Implement encoder settings in camerabin backend.

This is not comprehensive since different encoders have different names
and representations for many settings, but it should cover bit rate for
most encoders, and quality and encodingMode for a number of common
encoders.

Change-Id: I0ba4e70c2f234e0deaaa02bdecc0f5198122c1e9
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>