LV2 Support

This PR adds basic LV2 support to Mixxx. It integrates seamlessly with our current Effects Framework. Just install an LV2 plugin and it will show up when you cycle through the effects.

Currently it is only supporting plugins which handle stereo input/output audio samples and do not require any additional features. Some plugins which were tested by me: Calf Phaser, Calf Flanger, SWH Karaoke.
Translators possibly have a hard time knowing we are talking about plugins here. What about ``This plugin has features which are not yet supported.``