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/options.cpp

@@ -496,12 +496,28 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
     },
     {
         // spice2x_FpsOpposite
-        .title = "Show FPS/Clock top-left",
+        .title = "Show FPS/Clock top-left (DEPRECATED - use -fpslocation instead)",
         .name = "fpsflip",
-        .desc = "Show FPS / clock / timer overlay on the top left of the screen instead of the top right.",
+        .desc = "Show FPS / clock / timer overlay on the top left of the screen instead of the top right. "
+            "Deprecated - use -fpslocation instead.",
         .type = OptionType::Bool,
+        .hidden = true,
         .category = "Overlay",
     },
+    {
+        // FpsLocation
+        .title = "FPS/Clock Location",
+        .name = "fpslocation",
+        .desc = "Select which corner of the screen the FPS / clock / timer overlay appears in.",
+        .type = OptionType::Enum,
+        .category = "Overlay",
+        .elements = {
+            {"topright", ""},
+            {"topleft", ""},
+            {"bottomleft", ""},
+            {"bottomright", ""},
+        },
+    },
     {
         // spice2x_SubScreenAutoShow
         .title = "Auto Show Subscreen",