panel: Move statuses to the aggregate menu

panel: Move statuses to the aggregate menu

Swap out the implementation of SystemIndicator with a dummy,
and build the aggregate menu. At the same time, remove the
poweroff and login screen menus, as those were fake aggregate
menus beforehand.

We lose some flexibility as we lose session-mode-based menu
layout, but as each component of the aggregate menu is supposed
to be "smart" in response to updating itself when session
state changes, I believe it's better than a declarative model.

https://bugzilla.gnome.org/show_bug.cgi?id=705845