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/games/io.h

@@ -8,6 +8,7 @@ namespace games {
     namespace OverlayButtons {
         enum {
             Screenshot,
+            ToggleAllWindows,
             ToggleMainMenu,
             ToggleSubScreen,
             InsertCoin,
@@ -20,7 +21,7 @@ namespace games {
             ToggleControl,
             TogglePatchManager,
             ToggleScreenResize,
-            ToggleOverlay,
+            ToggleFps,
             ToggleCameraControl,
             TriggerPinMacroP1,
             TriggerPinMacroP2,