Add support for LFOs

[paulfd]

@jpcima already provided the LFO generation in #43 which allows to generate the waveforms from opcodes. What is needed now:

• [x] Plumbing to have Regions store this information (with quick tests)
• [x] Add either an LFOPool or a preallocation in the synth so that Voices can have the necessary allocated space to handle LFO operation
• [x] Add CC/pitch modulation to LFO targets
• [x] Add monophonic aftertouch event tracking in the midi state, LV2 and VST plugins
• [x] Add aftertouch modulation to LFO targets
• [ ] Add LFO modulation to LFO targets

Some questions remaining:

• Are LFOs on different voices/regions supposed to be synchronized somehow?
• Since LFOs can modulate LFOs, there needs to be a way to avoid cycles in this modulation graph. Either we force an evaluation order (the preferred, simpler approach) with a cycle check. Maybe verify/ask what other implementations are doing.

[jpcima]

LFOs synchronize with the start of note of their region. That includes their phase, their start delay, etc..

[AlexMitchellMus]

@paulfd there was a discussion to allow voice LFO's to have a free running setting.

I'm assuming that such setting would lock the LFO to global time. Hence it would act differently for LFOs defined by beat- or freq.

Obviously for free running defined LFOs, there would be no sync with start of note, and other LFO opcodes.

However free running phase would be possible. (Assuming a free running LFO would be 0° at time 0)