Add missing actor member on PopupMenu

[jp-vernooy]

The PopupMenu class has an actor member of type BoxPointer that's used when setting up a menu for a panel indicator. It's defined here: https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/main/js/ui/popupMenu.js?#L879

An example of this member being accessed from outside the class can be found here: https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/main/js/gdm/loginDialog.js#L337

When implementing a menu for an indicator, you would do something like this:

class MyIndicatorMenu extends PopupMenu.PopupMenu {
  constructor(sourceActor: St.Widget, arrowAlignment: number, arrowSide: St.Side) {
    super(sourceActor, arrowAlignment, arrowSide);

    Main.uiGroup.add_child(this.actor);
    this.actor.hide();

    this.addAction("Some action", () => { });
  }
}

Without this addition you have to resort to (this as any).actor in order to add the menu to uiGroup and hide it appropriately without type errors.

From the shell code, it doesn't seem like it's ever modified from the outside, so I've made it readonly.

EDIT: I've added the missing actor member to PopupDummyMenu and PopupMenuSection as well.

[jp-vernooy]

I see similar references being made to the actor member in PopupDummyMenu and PopupMenuSection. I suppose I could add those as well.

[Totto16]

Thank you for your contribution ❤️