panel: Add an easier way of adding items to the system status area

Extensions often want to add items to the system status area, so it
is useful to add a convenience API for it. Also, we now allow
for cleaner destruction of panel objects, by just calling destroy()
on it.
Based on a patch by Jasper St. Pierre.

https://bugzilla.gnome.org/show_bug.cgi?id=653205