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/window.cpp

@@ -3,6 +3,7 @@
 #include "util/logging.h"
 #include "games/io.h"
 #include "misc/eamuse.h"
+#include "external/imgui/imgui_internal.h"
 
 
 overlay::Window::Window(SpiceOverlay *overlay) : overlay(overlay) {
@@ -35,6 +36,12 @@ void overlay::Window::update() {
             } else {
                 this->toggle_active();
             }
+
+            // raise to the top, but only because the user pressed the hotkey and
+            // the window is now visible
+            if (this->active) {
+                this->bring_to_front();
+            }
         }
         this->toggle_button_state = toggle_button_new;
     }
@@ -81,14 +88,29 @@ void overlay::Window::build() {
             ImGui::PushStyleVar(ImGuiStyleVar_WindowBorderSize, 0.0f);
         }
 
-        // create window
-        if (ImGui::Begin(
-                (this->title + "###" + to_string(this)).c_str(),
-                &this->active,
-                this->flags)) {
+        const bool custom_window_padding =
+            !this->remove_window_padding && this->window_padding.x >= 0.f;
+        if (custom_window_padding) {
+            ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, this->window_padding);
+        }
 
-            // window attributes
-            this->calculate_initial_window();
+        // create window
+        // NoFocusOnAppearing: when the overlay is re-shown every window reappears at
+        // once and ImGui would auto-focus whichever is submitted last, stealing the
+        // top spot. suppress that so only an explicit focus request (see below)
+        // decides what comes to the front.
+        const std::string window_id = this->title + "###" + to_string(this);
+        if (ImGui::Begin(
+                window_id.c_str(),
+                &this->active,
+                this->flags | ImGuiWindowFlags_NoFocusOnAppearing)) {
+
+            // window attributes - init_pos / init_size are only honored once
+            // (ImGuiCond_Once), so compute them a single time instead of every frame
+            if (!this->initial_window_calculated) {
+                this->initial_window_calculated = true;
+                this->calculate_initial_window();
+            }
             ImGui::SetWindowPos(this->init_pos, ImGuiCond_Once);
             ImGui::SetWindowSize(this->init_size, ImGuiCond_Once);
 
@@ -103,12 +125,27 @@ void overlay::Window::build() {
         // end window
         ImGui::End();
 
+        // apply an explicit focus request now that the window exists. FocusWindow
+        // with UnlessBelowModal raises it to the front, but keeps it right below
+        // any blocking popup/modal instead of jumping in front of it (this also
+        // re-orders brand-new windows that ImGui places on top by default).
+        if (this->request_focus) {
+            this->request_focus = false;
+            if (ImGuiWindow *w = ImGui::FindWindowByName(window_id.c_str())) {
+                ImGui::FocusWindow(w, ImGuiFocusRequestFlags_UnlessBelowModal);
+            }
+        }
+
         if (this->remove_window_padding) {
             ImGui::PopStyleVar();
             ImGui::PopStyleVar();
             ImGui::PopStyleVar();
         }
 
+        if (custom_window_padding) {
+            ImGui::PopStyleVar();
+        }
+
     } else {
 
         // add raw content
@@ -142,6 +179,10 @@ void overlay::Window::set_active(bool active) {
     }
 }
 
+void overlay::Window::bring_to_front() {
+    this->request_focus = true;
+}
+
 bool overlay::Window::get_active() {
 
     // check for active children