popupMenu: Introduce a way of closing toplevels from sections

As the aggregate menu will be built out of sections from each
of the menus, we need to ensure that activating an item in one
of these sections can close the main menu, even when it is not
a menu item. The new API also needs to be flexible enough to
ensure that animations can be controlled, like the buttons that
lock the screen or launch a new session.

Port the user menu to use this new API as well.

https://bugzilla.gnome.org/show_bug.cgi?id=702539