Fixes seeking in FFMPEG/Avconv from non-working state to Mixxx needed accurate seeking

Version should work on FFMPEG 0.11 - 2.2 and Avconv 0.8 - 11 (So Debian is supported also old LTS Ubuntu). Seeking in some cases is slow as mules a** but this could be fixed in future. There is small cache for frames because to make backward seeking more effience and also code uses jump points to make seeking little bit faster.

There is still known memory leak that I'm tracking down but otherwise this should be tested and see if it fixes anything.