GStreamer: don't try to seek when it's not supported.

This eliminates warnings on the console.

Change-Id: I6a0509dba4a0f7ec6fad33f8803be746f425a616
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
GStreamer: fix memory leaks.

Many GStreamer objects were not properly managed or never released.

Change-Id: I38b3854e8b9e2264b5b647f331d3bb16b886e2d6
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>
Improved declarative-camera example.

- Fix setting the white balance preset, it was never actually set on the
  camera
- Improved the zoom control style

Change-Id: I95c2851e5ad8409dd79306492271f221a61cb10d
Reviewed-by: Christian Stromme <christian.stromme@digia.com>