Have a global timer.  Update all visible terminal windows syncronously.
Right now this can be the active_console only, but that will change
soon.  The global timer will disable itself if not needed, so we only
have to care start it if needed.  Which might be at console switch time
or when a new displaychangelistener is registered.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>