messageTray: Don't exit out early if we have a left timeout

Otherwise, we won't mark the pointer as hovering on the notification.

https://bugzilla.gnome.org/show_bug.cgi?id=731118
messageTray: Squash together some duplicated code

https://bugzilla.gnome.org/show_bug.cgi?id=731118
messageTray: Clear the pointer left timeout when showing a notification

When the pointer leaves the notification area, we queue a timeout to
hide the notification after a little while. If the user is hovering over
a notification and clicks the X button to close the notification, we will
destroy the notification, which causes a "pointer left" event on the
notification area. This queues a timeout which erroneously fires after
the next notification in the queue shows up.

The code and state machine are too complex to properly make sure this
timeout doesn't fire when there is no notification up next, so instead
just clear it when showing a notification to make sure that any
previously queued timeout doesn't apply to us.

https://bugzilla.gnome.org/show_bug.cgi?id=731118