Fixed envelope sustain response
Renamed PlayHandle classes and some functions with bool return values

Next big coding style update - this time all PlayHandle classes are
affected. Functions like done() and released() were renamed to
isFinished() and isReleased().
NotePlayHandle, InstrumentTrack: removed obsolete singerbot support

The singerbot plugin doesn't exist anymore and thus any support in the
code base just adds complexity and confusion, therefore remove it.
RemoteVstPlugin: fixed handling of MIDI pitch bend events

Fix a regression introduced with commit 94f991848e6a3788679018fef64c1f9.

Closes #172.
More envelope sustain respon fixes
Envelope graph shooting over target
Bit Invader smooth button active and inactive graphic
Merge pull request #199 from wongcc966422/bitInvaderSmoothGraphic

Bit Invader: added active and inactive graphic for smooth button
Merge pull request #191 from zonkmachine/stable-0.4

Fixed envelope sustain response
Sfxr artwork updates