Updated; includes better error handling and color balance control.

The program does not instantly quit on GStreamer errors anymore.

Add color balance control. For now only for GStreamer 1.0 with GTK+
3 (support for this feature with GTK+ 2 is feasable, but support with
GStreamer 0.10 would require more work).

There is a configuration interface which should eventually allow global
and uri-specific settings for color balance and audio volume to be
saved (not yet implemented).