Fix xvimagesink color balance control for certain configurations

In certain GStreamer 1.0 installations, there is a mismatch between
the color channel names reported by the GstColorBalance interface
(e.g. "BRIGHTNESS") and the names actually expected by xvimagesink
(e.g. "XV_BRIGHTNESS"). This commit implements a work around by
detecting the use of xvimagesink in the pipeline and making sure
the channel names start with "XV_".

Color balance control with GStreamer 0.10 now also works.

Color balance control is currently only supported with GTK+ 3.0,
but this limitation could easily be removed.