Use gstreamer under the hood

[exzombie]

When qarv was first written, gstreamer-0.10 was used in Qt. Because gstreamer does not support linking to multiple versions of the library, we could not make use of gstreamer-1.0 which had features that we wanted (although I can't remeber what they were after all this time). So we went our own way. Things are different now and we could probably make our lives easier by ditching the Q_SIGNAL based transport and custom widgets. Or not, it remains to be seen, but we should definitely check it out. In any case, it would be nice not to call into ffmpeg directly as it changes API rather often. It would also be nice if qarv could be used as a gstreamer source, or maybe a control panel for gstaravis.