We can't silently replace the old behavior of separate status
icons into a new system. Replace SystemStatusButton with a new
SystemIndicator class which will allow for the flexibility we
need. For now, make it a subclass of Button so that it mostly
feels the same, but we'll soon be swapping it out with a dummy
implementation that the aggregate menu will use.
I think the code cleanup here is worth it.
https://bugzilla.gnome.org/show_bug.cgi?id=705845
5cca26a565
