Merge branch 'swesterfeld-flexible-adsr', closes tim-janik#6

* swesterfeld-flexible-adsr:
  DEVICES: blepsynth/blepsynth.cc: cleanup unneccessary initialization
  DEVICES: blepsynth/blepsynth.cc: introduce flexible ADSR model
	 - volume envelope can now be analog (exponential) or flexible
	 - flexible envelope has adjustable attack/decay/release slope
	 - support changing volume/filter envelope params while note is playing

Signed-off-by: Tim Janik <[email protected]>