Adwaita: Give model buttons visible focus

Hard to use keynav, otherwise.  Also, hard
to debug focus issues if you can't see where
the focus is.

gtk/theme/Adwaita/_common.scss

@@ -16,6 +16,7 @@ $window_radius: $button_radius + 3;
   -gtk-secondary-caret-color: $selected_bg_color
 }
 
+modelbutton:focus(visible),
 button:focus(visible),
 checkbutton:focus(visible),
 radiobutton:focus(visible),