Modal stack: fix handling of destroyed actors

Destroyed modal actors should be completely removed from the modal
stack automatically, including leaving modality if needed.
This allows for destroying modal dialogs without calling close().

js/ui/main.js

@@ -668,7 +668,7 @@ function pushModal(actor, timestamp, options) {
     let actorDestroyId = actor.connect('destroy', function() {
         let index = _findModal(actor);
         if (index >= 0)
-            modalActorFocusStack.splice(index, 1);
+            popModal(actor);
     });
     let curFocus = global.stage.get_key_focus();
     let curFocusDestroyId;