[Player] Update UI based only on gstreamer state
[Player] Update UI based only on gstreamer state

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

Signed-off-by: Jussi Kukkonen <589e25658b1bfd3646cd0c60eea9e9594b5e0b5b@intel.com>

jku/media-service-demo