pushModal: clear focus after pushing a new level

This way the new modal level is independent of whatever may have been
happening before. Fixes a problem with status menus becoming active
again from inside the app switcher, etc.

js/ui/main.js

@@ -390,6 +390,7 @@ function pushModal(actor) {
     }
     modalActorFocusStack.push([actor, curFocus]);
 
+    global.stage.set_key_focus(null);
     return true;
 }