modalDialog: Make reactive

Some dialogs like the runDialog expect this actor to receive
key events while it is not reactive. Whatever that black magic was
it will no longer work.

Make the actor reactive, so it can simply handle key events.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2045>

js/ui/modalDialog.js

@@ -32,9 +32,11 @@ var ModalDialog = GObject.registerClass({
 }, class ModalDialog extends St.Widget {
     _init(params) {
         super._init({ visible: false,
-                      x: 0,
-                      y: 0,
-                      accessible_role: Atk.Role.DIALOG });
+            reactive: true,
+            x: 0,
+            y: 0,
+            accessible_role: Atk.Role.DIALOG,
+        });
 
         params = Params.parse(params, { shellReactive: false,
                                         styleClass: null,