overlay: refactor overlay layering (#739)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change

Create three distinct layers for the overlay:

1. Bottommost persistent layer - non-interactable layer that is always
on. This was only for notifications, but now the FPS widget lives here
as well.
2. Overlay windows layer - most interactable windows go here.
3. Topmost main menu - this is reserved for the main menu (escape key)
and this is a modal dialog that occludes the layers below.

Why? 

- `toggle overlay` behavior with FPS widget *also* toggling on/off was a
bit confusing (now they're two separate keys)
- FPS widget is popular, but it caused the entire overlay to be active,
which affects how input is handled
- the main menu being a standalone window was a little awkward (now it's
a modal)

## Testing

src/spice2x/overlay/windows/exitprompt.cpp

@@ -1,5 +1,6 @@
 #include "exitprompt.h"
 #include "avs/game.h"
+#include "build/defs.h"
 #include "misc/eamuse.h"
 #include "util/logging.h"
 #include "games/iidx/iidx.h"
@@ -9,7 +10,7 @@
 namespace overlay::windows {
 
     ExitPrompt::ExitPrompt(SpiceOverlay *overlay) : Window(overlay) {
-        this->title = "spice2x";
+        this->title = "spice2x (" + to_string(VERSION_STRING_CFG) + ")";
         this->init_size = ImVec2(
             (ImGui::GetFontSize() * 14) + (ImGui::GetStyle().ItemSpacing.x * 2),
             overlay::apply_scaling(120));
@@ -20,22 +21,35 @@ namespace overlay::windows {
         this->flags = ImGuiWindowFlags_NoResize
                       | ImGuiWindowFlags_NoCollapse
                       | ImGuiWindowFlags_AlwaysAutoResize
-                      | ImGuiWindowFlags_NoDocking;
+                      | ImGuiWindowFlags_NoDocking
+                      | ImGuiWindowFlags_NoMove;
+
+        // the menu renders itself as a popup, not a normal overlay window
+        this->draws_window = false;
+    }
+
+    void ExitPrompt::update() {
+        Window::update();
+
+        // allow the popup to be reopened the next time the menu is shown
+        if (!this->active) {
+            this->popup_opened = false;
+        }
     }
 
     void ExitPrompt::build_button(
-        Window *window, std::string label, const ImVec2 &size, NextItem next, bool is_toggle) {
+        Window *window, std::string label, const ImVec2 &size, NextItem next) {
 
         if (window == nullptr) {
             return;
         }
 
         if (ImGui::Button(label.c_str(), size)) {
-            if (is_toggle) {
-                window->toggle_active();
-            } else {
-                window->set_active(true);
-                this->set_active(false);
+            window->toggle_active();
+
+            // raise to the top when the user toggled it visible from the menu
+            if (window->get_active()) {
+                window->bring_to_front();
             }
         }
         if (next == NextItem::NEW_LINE) {
@@ -46,6 +60,31 @@ namespace overlay::windows {
     }
 
     void ExitPrompt::build_content() {
+        // use ### so the visible label is the full title (with version) while the
+        // popup keeps a stable identifier regardless of the title text
+        const std::string popup_id_str = this->title + "###mainmenu";
+        const char *popup_id = popup_id_str.c_str();
+
+        // open the popup once when the menu becomes active; popup_opened is reset
+        // in update() while inactive so it reopens the next time it is shown
+        if (!this->popup_opened) {
+            ImGui::OpenPopup(popup_id);
+            this->popup_opened = true;
+        }
+
+        // position at the top center of the screen
+        const ImVec2 menu_pos(
+            ImGui::GetIO().DisplaySize.x / 2 - this->init_size.x / 2,
+            overlay::apply_scaling(10));
+        ImGui::SetNextWindowPos(menu_pos, ImGuiCond_Appearing);
+
+        bool open = true;
+        if (!ImGui::BeginPopupModal(popup_id, &open, this->flags)) {
+            // dismissed via escape - close the menu
+            this->set_active(false);
+            return;
+        }
+
         const ImVec2 size(ImGui::GetFontSize() * 14, ImGui::GetFontSize() * 1.9f);
         const ImVec2 size_half(
             (size.x - ImGui::GetStyle().ItemSpacing.x) / 2,
@@ -54,11 +93,7 @@ namespace overlay::windows {
             (size.x - (ImGui::GetStyle().ItemSpacing.x * 2)) / 3,
             ImGui::GetFontSize() * 2.5f);
 
-        if (ImGui::Button("Hide overlay", size)) {
-            overlay::OVERLAY->set_active(false);
-        }
-        ImGui::Spacing();
-        build_button(this->overlay->window_config, "Show Config", size, NextItem::NEW_LINE, false);
+        build_button(this->overlay->window_config, "Options", size, NextItem::NEW_LINE);
 
         std::string sub = "Show Subscreen";
         if (avs::game::is_model("LDJ")) {
@@ -77,11 +112,12 @@ namespace overlay::windows {
             sub = "Show Pop'n Subscreen";
         }
 
-        build_button(this->overlay->window_sub, sub, size, NextItem::NEW_LINE, false);
+        build_button(this->overlay->window_sub, sub, size, NextItem::NEW_LINE);
 
         ImGui::TextDisabled("Graphics");
         build_button(this->overlay->window_camera, "Camera control", size, NextItem::NEW_LINE);
-        build_button(this->overlay->window_fps, "FPS", size_half, NextItem::SAME_LINE);
+
+        build_button(this->overlay->window_fps.get(), "FPS", size_half, NextItem::SAME_LINE);
         build_button(this->overlay->window_resize, "Resize", size_half, NextItem::NEW_LINE);
 
         ImGui::TextDisabled("I/O");
@@ -157,5 +193,12 @@ namespace overlay::windows {
             ImGui::EndPopup();
         }
         ImGui::Spacing();
+
+        ImGui::EndPopup();
+
+        // title-bar X was clicked - close the menu
+        if (!open) {
+            this->set_active(false);
+        }
     }
 }