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.h

@@ -14,8 +14,13 @@ namespace overlay::windows {
     public:
         ExitPrompt(SpiceOverlay *overlay);
         void build_content() override;
+        void update() override;
 
     private:
-        void build_button(Window *window, std::string label, const ImVec2 &size, NextItem next, bool is_toggle=true);
+        void build_button(Window *window, std::string label, const ImVec2 &size, NextItem next);
+
+        // latch so the popup is opened once per activation; reset in update()
+        // whenever the menu is inactive
+        bool popup_opened = false;
     };
 }