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/notifications.cpp

@@ -146,6 +146,17 @@ namespace overlay::notifications {
 
         float height = 0.f;
         if (ImGui::Begin(window_id.c_str(), nullptr, TOAST_FLAGS)) {
+            // keep toasts above other overlay windows (e.g. the persistent FPS
+            // window, which may be toggled on after a toast already exists), but
+            // tuck them behind a blocking modal so they get dimmed/occluded by the
+            // modal backdrop instead of floating on top of it.
+            ImGuiWindow *toast_window = ImGui::GetCurrentWindow();
+            if (ImGuiWindow *modal = ImGui::GetTopMostPopupModal()) {
+                ImGui::BringWindowToDisplayBehind(toast_window, modal);
+            } else {
+                ImGui::BringWindowToDisplayFront(toast_window);
+            }
+
             const ImVec2 win_pos = ImGui::GetWindowPos();
             const ImVec2 win_size = ImGui::GetWindowSize();