shellDBus: Emit a PropertyChanged for OverviewActive

I was going to add another DBus property to signal when the shell was
done loading and was idle, and while implementing that I noticed we
aren't emitting PropertyChanged for, well, any property.  Let's fix
OverviewActive.

It's unfortunate it's so tedious to correctly implement a DBus
property =/

https://bugzilla.gnome.org/show_bug.cgi?id=704163