fixed a bug with incorrect handling of the BACK button/gesture

bug fixes

* new option - Hold; if enabled, notes will play endlessly without reacting to key release (noteOff) events; turning the note on again works like turning it off; there are two ways to enable this option:
  1) using the Hold parameter in the "MIDI Mapping" window;
  2) by pressing the HOLD button, which appears in place of LCK while playing the musical keyboard: press HOLD, release the desired notes - then the released notes will continue to play;
* additional highlighting of locked samples;
* bug fixes.