legacyTray: Add a hideable tray for legacy status icons

legacyTray: Add a hideable tray for legacy status icons

Commit 5a8923ef95a2 removed support for legacy status icons from
the notification system, as we no longer want them to appear as
notifications. As we are unfortunately not quite at a point where
we can remove all support for them for good, so we now need an
alternative place to put them. Add a small dedicated tray at the
bottom which appears when any legacy status icons are active. By
default it is almost completely hidden to not interfere with the
user's windows, but can be expanded on demand to interact with
the icons.

https://bugzilla.gnome.org/show_bug.cgi?id=745162