feat(tracker): add menu to load instrument presets

The presets are embedded in the executable, so there's no additional files.

Closes #91

tracker/presets.go

@@ -0,0 +1,51 @@
+package tracker
+
+import (
+	"embed"
+	"io/fs"
+	"sort"
+
+	"github.com/vsariola/sointu"
+	"gopkg.in/yaml.v3"
+)
+
+type PresetList []sointu.Instrument
+
+//go:embed presets/*
+var presetFS embed.FS
+
+var Presets PresetList
+
+func init() {
+	fs.WalkDir(presetFS, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			return nil
+		}
+		data, err := fs.ReadFile(presetFS, path)
+		if err != nil {
+			return nil
+		}
+		var instr sointu.Instrument
+		if yaml.Unmarshal(data, &instr) != nil {
+			return nil
+		}
+		Presets = append(Presets, instr)
+		return nil
+	})
+	sort.Sort(Presets)
+}
+
+func (p PresetList) Len() int {
+	return len(p)
+}
+
+func (p PresetList) Less(i, j int) bool {
+	return p[i].Name < p[j].Name
+}
+
+func (p PresetList) Swap(i, j int) {
+	p[i], p[j] = p[j], p[i]
+}