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.
I must have missed adding samplerate COs to GroupEffectProcessor (I think it may introduce less coupling between the engine and the effects system if the current samplerate is passed into the initialize() and process() methods) but the delete in the destructor (a few lines down) should be moved into the EffectProcessor destructor.