[player] Use buttons with stock media images

Also use a play/pause button instead of separate ones

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[Player] Remove Stop-button

Remove "Stop" button: it does not give the user much added value
and the relation to pause is not clear to many people (this is
why most modern players do not use it).

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[Player] Update UI based only on gstreamer state

This is simpler and less error-prone than doing it both from
gstreamer and UI event handlers.

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