overlay: re-enable keyboard navigation (#802)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #800 

## Description of change
Restores keyboard navigation by default in spicecfg, and add an option
to optionally enable it in the overlay.

## Testing

src/spice2x/launcher/options.cpp

@@ -510,6 +510,13 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
         .type = OptionType::Bool,
         .category = "General Overlay",
     },
+    {
+        .title = "Overlay Keyboard Navigation",
+        .name = "keyboardnav",
+        .desc = "Enables keyboard navigation in the in-game overlay.",
+        .type = OptionType::Bool,
+        .category = "General Overlay",
+    },
     {
         // OverlayScaling
         .title = "Spice Overlay UI Scale %",