We use two windows in GtkViewport style to animate the drop down.

Also fixes some issues:
 * Don't use first draw to trigger, instead use visibility
   events which seems cleaner
 * Rename timeout signal to dismissed, because we need to
   actually delete the contact both on timeout and if
   manually dismissed
 * Also dismiss on destroy, as we want to delete if the window
   is closed while a notification is showing.
 * Use gdk_thread_add_timeout for correct threadsafety handling