Wait on queue condition

Commit 9f0441a ("Move sending pause/seek requests into the thread to
avoid problems") dropped the vlc_cond_wait() call, resulting in a busy
loop when waiting for new messages.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Merge pull request #8 from swegener/for-upstream

Wait on queue condition