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.cpp

@@ -444,6 +444,8 @@ namespace games {
         // overlay button definitions
         names.emplace_back("Screenshot");
         vkey_defaults.push_back(VK_SNAPSHOT);
+        names.emplace_back("Toggle All Windows");
+        vkey_defaults.push_back(VK_OEM_3); // backtick `
         names.emplace_back("Toggle Main Menu");
         vkey_defaults.push_back(VK_ESCAPE);
         names.emplace_back("Toggle Sub Screen");
@@ -468,7 +470,7 @@ namespace games {
         vkey_defaults.push_back(VK_F10);
         names.emplace_back("Toggle Screen Resize");
         vkey_defaults.push_back(VK_F11);
-        names.emplace_back("Toggle Overlay");
+        names.emplace_back("Toggle FPS");
         vkey_defaults.push_back(VK_F12);
         names.emplace_back("Toggle Camera Control");
         vkey_defaults.push_back(0xFF);