feat: add MCHD command

[ax0]

Description

This PR adds the MCHD command, which opens the door to chords for MIDI instruments. In addition, muting and unmuting of MIDI channels now works: Muting immediately cuts off the last note and discards all MIDI notes and commands until the channel is unmuted.

Type of change

• Resolves Channel mutes does nothing on MIDI instruments #104

• Partially resolves ARPG does nothing on MIDI instruments #103

• Bug fix (non-breaking change which fixes an issue)

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Use the MCHD aabb command on a MIDI instrument with various values of aa and bb. Note that the notes aa and bb semitones above the last played note (modulo 128) are played. These notes should last as long as the 'root note'.
• Mute and unmute a MIDI instrument channel, noting that notes stop sounding and commands stop getting sent.

Test Configuration:

• Hardware: x64, aarch64 Linux
• Test steps:
  See above.

Checklist:

• I have performed a self-review of my code
• I have commented particularly in hard-to-understand areas
• I have updated CHANGELOG
• I have updated docs/wiki/What-is-LittlePiggyTracker.md reflecting my changes
• I have version bumped in sources/Application/Model/Project.h
• My changes generate no new warnings (build without your change then apply your change to check this)
• No LLMs or other sources or tools that may call this project's integrity or license into question have been used or consulted