Add QT_GSTREAMER_CAMERABIN_VIDEOSRC environment variable.

It can be used to set which video source element should be used by
the camerabin.

Change-Id: I8d1cd8c4ba6fe5a89817699f645b0997e713aaca
Reviewed-by: Samuli Piippo <samuli.piippo@digia.com>
Reviewed-by: Christian Stromme <christian.stromme@digia.com>