Previously, every time _updateState was called, it would make some
change, and so it was necessary to very carefully set up all the calls
to it, to ensure it was always called at exactly the right time. Now,
instead, we keep a bunch of state variables like "_notificationState"
and "_pointerInSummary", and potentially multiple timeouts, and
_updateState looks at all of them and figure out what, if anything,
needs to be changed.
By making the rules about what causes changes more explicit, it will
be easier to change those rules in the future as we add new
functionality.
Also, update the rules a bit, so that notifications can appear while
the summary is visible, and the summary only shows after a
notification if the summary has changed.
https://bugzilla.gnome.org/show_bug.cgi?id=609765
e86c821878