When GLArea gets a GL error it leaves user with nothing, but transparent background with error text in the middle. Make it look somewhat decent by drawing black background for the not working video widget.
The "clapper_src" directory name was unusual. This was done to make it work as a widget for other apps. Now that this functionality got removed it can be named simply "src" as recommended by guidelines.
Default "seek" function takes time in nanoseconds as argument which is not that useful, cause we will start playback from the nearest keyframe anyway. The new "seek_seconds" can take a double value for more precise seeking.
