tests/examples/seek/seek.c: Rearrange some buttons in playbin2 and make some other boxes insensitive when needed.

Original commit message from CVS:
* tests/examples/seek/seek.c: (audio_toggle_cb), (video_toggle_cb),
(text_toggle_cb), (update_streams), (main):
Rearrange some buttons in playbin2 and make some other boxes insensitive
when needed.
Add language codes to subtitle selection boxes when we gind the right
tags for the streams.