Clean up GStreamer libraries
Since (the nightlies of) December 6th, Gajim uses GStreamer for (audio) previews and possibly future AV support. On Windows, the libraries included now account for a size increase of at least 200MB (which in itself is 20MB more than the entire Gajim folder of the previous version).
Most codecs currently included are of no use whatsoever, so considering the fact that Gajim's program folder in Windows is now 2.7x larger than the previous version, I think it would be wise to tailor the included codecs to what Gajim will actually use.