messageTray: Move adding the menu actor to MessageTrayContextMenu

This matches BackgroundMenu and EntryMenu.

https://bugzilla.gnome.org/show_bug.cgi?id=699272
messageTray: Use a PopupMenuManager for the message tray context menu

https://bugzilla.gnome.org/show_bug.cgi?id=699272
messageTray: Rename the context menu to the tray menu

https://bugzilla.gnome.org/show_bug.cgi?id=699272