LV2 Support

This is an updated version of https://github.com/mixxxdj/mixxx/pull/316 . It uses @daschuer's version of handling effect parameters (a common list for both button and knob parameters).

For further comments, check the old PRhttps://github.com/mixxxdj/mixxx/pull/316] and the wiki: http://www.mixxx.org/wiki/doku.php/lv2_support