[STI] add optional note-hint metadata

[bazz1tv]

Embed the note hint into the instrument file format as optional metadata. It should be written as a string to avoid implementation changes down the line. The following format can be used:

[A-G]#{0,1}(-[1-2]|[0-6] Corner cases: Only C6 for 6th octave. No sharps allowed on B, E. Negative octaves aren't yet usable in app but are valid.

Examples:

C4
A3
B0
C6
D#4
E-1

[RowanDDR]

Riffing off this idea, as well as note hint, perhaps the STI metadata could also contain an optional relative pitch number. e.g. how many semitones (and finetune steps) above/below a concert pitch "C3". This could lend itself to future features regarding tuning or auto tuning of instruments etc.