feat(sointu, tracker,...): restructure domain & tracker models

send targets are now by ID and Song has "Score" part, which is the notes for it. also, moved the model part separate of the actual gioui dependend stuff.

sorry to my future self about the code bomb; ended up too far and did not find an easy way to rewrite the history to make the steps smaller, so in the end, just squashed everything.

instrument.go

@@ -0,0 +1,16 @@
+package sointu
+
+// Instrument includes a list of units consisting of the instrument, and the number of polyphonic voices for this instrument
+type Instrument struct {
+	Name      string `yaml:",omitempty"`
+	NumVoices int
+	Units     []Unit
+}
+
+func (instr *Instrument) Copy() Instrument {
+	units := make([]Unit, len(instr.Units))
+	for i, u := range instr.Units {
+		units[i] = u.Copy()
+	}
+	return Instrument{Name: instr.Name, NumVoices: instr.NumVoices, Units: units}
+}