[Player] Fix seeking

Do seeks so that the UI doesn't block. Enable seeking
while playing.

Fixes #3

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[Player] Create the adjustment like other widgets

Create the adjustment on init(), only do set_upper()
when duration is available.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[Player] Move play-pause-button next to the progress scale

Change is a purely visual one.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>