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.h

@@ -53,6 +53,7 @@ namespace launcher {
             SOFTID,
             VREnable,
             DisableOverlay,
+            OverlayKeyboardNavigation,
             OverlayScaling,
             NotificationPosition,
             spice2x_FpsAutoShow,