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;

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",

src/spice2x/launcher/options.h

@@ -56,6 +56,7 @@ namespace launcher {
             NotificationPosition,
             spice2x_FpsAutoShow,
             spice2x_FpsOpposite,
+            FpsLocation,
             spice2x_SubScreenAutoShow,
             spice2x_IOPanelAutoShow,
             spice2x_KeypadAutoShow,