tests/examples/seek/seek.c: Add combo box for visualisations, populate it with a factory list of all visualisation pl...

Original commit message from CVS:
* tests/examples/seek/seek.c: (vis_toggle_cb), (filter_features),
(init_visualization_features), (vis_combo_cb), (shot_cb), (main):
Add combo box for visualisations, populate it with a factory list
of all visualisation plugins, configure vis plugin instance in
playbin2.