Blepsynth improvements

[swesterfeld]

Here is a bunch of new stuff for BlepSynth, I won't try to reproduce the commit entries here. Only a few additional observations:

• x^3 non-linear adsr time scaling used to work but now it doesn't probably due to parameter API changes (I didn't try to fix it)
• some parameters (octave, unison voices) need step=1 but I haven't seen how to set it when registering a parameter, so this workarounded a bit by overriding param-to-text conversion and omitting decimals
• new filter envelope is hard-wired to modulate cutoff, whereas it would make sense if it could modulate any parameter, but we don't have the ui for selecting modulation targets yet (nor the infrastructure bits)

[tim-janik]

Thanks, its merged now, except for triggering C-G, these still help with testing atm.