LV2 support

This pull request brings (initial) LV2 support for Non-Mixer.

Still a bit limited, there are no features implemented besides LV2-core.
Having custom UIs is not something that can be easily done with NTK, and I'm not much interested on it right now.
We can go on a per-plugin basis, getting the needed features working as needed.

The plugin chooser dialog needs some updating, it currenly only lists LADSPA in the combo-box, I'll leave that to you.
The code style doesn't always matches yours, sorry. If this is a big issue I can update it.