tests/examples/seek/scrubby.c: Set higher priority for bus events so they don't get reordered with gtk gui events.

Original commit message from CVS:
* tests/examples/seek/scrubby.c: (main):
Set higher priority for bus events so they don't get reordered with
gtk gui events.

* tests/examples/seek/seek.c: (do_seek), (start_seek), (stop_seek),
(flush_toggle_cb), (main):
Added checkbox do disable flushing seeks.
Disable scrubbing when doing non flushing seeks.