messageTray: Hook SourceActor up to source icon changes automatically

Instead of manually tracking source icon changes, or requiring a manual
call to _setSummaryIcon, add a way to emit a signal when we're guaranteed
the icon has been changed, and then the source actor will automatically
update the icon.
_setSummaryIcon is still available for sources such as the notification
daemon, that require special treatment for the summary icon (to be used
with tray icons)

https://bugzilla.gnome.org/show_bug.cgi?id=680426