Adds an ADSR envelope for controlling how MIDI notes are played

[jameshball]

• ADSR (Attack, Decay, Sustain, Release) envelope added in MIDI section
• Points on the envelope can be dragged around to change the dynamics of notes played
• You can drag the curve itself to change its shape
• Doesn't affect anything if Enable MIDI is disabled

STILL TODO:

• Make it look good
• Add meaningful labels on the points of the graph
  • i.e. Attack, Decay, etc.
• Add a grid so you can more precisely time it