Release MK Slicer v3.21 (#1297)

+ Theme: Checkboxes (operating mode switches) gets a color tint that reflects the active mode.
+ Theme: The toggle buttons at the top are now colored when turned on.
+ Theme: Now the top panel and BPM indicator have a customizable background.
+ Theme: Symbols on the Settings and Snap buttons have been replaced with graphics.
+ Fixed a bug: now, when the Velocity Range slider is in extreme positions, the velocity points do not go beyond the boundaries of the window.
+ Fixed a bug: now BPM is not displayed if an unrealistically large value is calculated.
+ Fixed a bug: now the DestroyAudioAccessor function works correctly.
+ Now the script can receive MIDI from any channel, not just the first.
+ Fixed typo: RealmGUI -> ReaimGUI