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/launcher/launcher.cpp

@@ -1182,7 +1182,20 @@ int main_implementation(int argc, char *argv[]) {
         overlay::AUTO_SHOW_FPS = true;
     }
     if (options[launcher::Options::spice2x_FpsOpposite].value_bool()) {
-        overlay::FPS_SHOULD_FLIP = true;
+        // deprecated flag: equivalent to anchoring the FPS window top-left
+        overlay::FPS_LOCATION = overlay::FpsLocation::TopLeft;
+    }
+    if (options[launcher::Options::FpsLocation].is_active()) {
+        const auto txt = options[launcher::Options::FpsLocation].value_text();
+        if (txt == "topright") {
+            overlay::FPS_LOCATION = overlay::FpsLocation::TopRight;
+        } else if (txt == "topleft") {
+            overlay::FPS_LOCATION = overlay::FpsLocation::TopLeft;
+        } else if (txt == "bottomleft") {
+            overlay::FPS_LOCATION = overlay::FpsLocation::BottomLeft;
+        } else if (txt == "bottomright") {
+            overlay::FPS_LOCATION = overlay::FpsLocation::BottomRight;
+        }
     }
     if (options[launcher::Options::spice2x_SubScreenAutoShow].value_bool()) {
         overlay::AUTO_SHOW_SUBSCREEN = true;