Channel Mutes

[zuggamasta]

Add solo / mute mode for performance.

• Basic muting function
• UX/UI that works with the rest of midiTracker
• Don't overwrite note velocities with '120' when unmuting

[zuggamasta]

First initial version using the midi velocity in commit e035ac1

the muting with the letters, A, S, D, F, G, H, J, K, which are all next to each other on the keyboard feels very clumsy. And hard to understand. the number would probably work better but are currently breaking how the rest of the tracker works.