dialogs: Re-enable panels on destroy

If we only do it on close() we're missing some use cases.

For instance:

- Right-click an applet
- Choose to remove it
- Click No.

Before this commit, the panel wasn't coming back.

js/ui/modalDialog.js

@@ -84,6 +84,8 @@ var ModalDialog = GObject.registerClass({
         this._cinnamonReactive = params.cinnamonReactive;
         this._destroyOnClose = params.destroyOnClose;
 
+        this.connect('destroy', this._onDestroy.bind(this));
+
         Main.layoutManager.modalDialogGroup.add_child(this);
 
         let constraint = new Clutter.BindConstraint({
@@ -369,6 +371,16 @@ var ModalDialog = GObject.registerClass({
             }
         });
     }
+
+    /**
+     * _onDestroy:
+     *
+     * This is called when the dialog actor is destroyed, either
+     * by destroying its container or by explicitly calling this.destroy().
+     */
+    _onDestroy() {
+        Main.panelManager.enablePanels();
+    }
 });
 
 /**