LV2 support

[swesterfeld]

I've been working on LV2 support. This is work in progress, so I'm submitting the current version here, while I know the code is somewhat incomplete. As discussed previously, there is no custom UI support yet, and should be added once we have out-of-process support.

• the ringbuffer is a copy from jack driver, so probably we should have Bse::FrameRingBuffer and Bse::RingBuffer implemented in bse/ringbuffer.hh and used by both
• the code is used by specifying at LV2 URI (defaults to zynaddsubfx), like

LV2URI=http://synthv1.sourceforge.net/lv2 make run # synth
LV2URI=http://calf.sourceforge.net/plugins/Monosynth make run # synth
LV2URI=urn:ardour:a-reverb make run # stereo effect

• preset loading should not be done in RT thread (as discussed in last meeting), and notify UI, too
• there should be a way to select which plugin to use (LV2URI) other than commandline