feat(tracker): add step parameter to advance cursor after edit

The step is currently in the song panel; might consider putting it somewhere else.

Closes #6

tracker/keyevent.go

@@ -211,7 +211,11 @@ func (t *Tracker) KeyEvent(w *app.Window, e key.Event) bool {
 				if e.Modifiers.Contain(key.ModShortcut) {
 					t.Cursor.Row -= t.song.RowsPerPattern
 				} else {
-					t.Cursor.Row--
+					if t.Step.Value > 0 {
+						t.Cursor.Row -= t.Step.Value
+					} else {
+						t.Cursor.Row--
+					}
 				}
 				t.NoteTracking = false
 			case EditUnits:
@@ -237,7 +241,11 @@ func (t *Tracker) KeyEvent(w *app.Window, e key.Event) bool {
 				if e.Modifiers.Contain(key.ModShortcut) {
 					t.Cursor.Row += t.song.RowsPerPattern
 				} else {
-					t.Cursor.Row++
+					if t.Step.Value > 0 {
+						t.Cursor.Row += t.Step.Value
+					} else {
+						t.Cursor.Row++
+					}
 				}
 				t.NoteTracking = false
 			case EditUnits:

tracker/songpanel.go

@@ -175,6 +175,19 @@ func (t *Tracker) layoutSongOptions(gtx C) D {
 				}),
 			)
 		}),
+		layout.Rigid(func(gtx C) D {
+			return layout.Flex{Axis: layout.Horizontal}.Layout(gtx,
+				layout.Rigid(Label("STP:", white)),
+				layout.Rigid(func(gtx layout.Context) layout.Dimensions {
+					numStyle := NumericUpDown(t.Theme, t.Step, 0, 8)
+					numStyle.UnitsPerStep = unit.Dp(20)
+					gtx.Constraints.Min.Y = gtx.Px(unit.Dp(20))
+					gtx.Constraints.Min.X = gtx.Px(unit.Dp(70))
+					dims := in.Layout(gtx, numStyle.Layout)
+					return dims
+				}),
+			)
+		}),
 		layout.Rigid(func(gtx C) D {
 			gtx.Constraints.Min = image.Pt(0, 0)
 			return panicBtnStyle.Layout(gtx)

tracker/tracker.go

@@ -47,6 +47,7 @@ type Tracker struct {
 	BPM                   *NumberInput
 	RowsPerPattern        *NumberInput
 	RowsPerBeat           *NumberInput
+	Step                  *NumberInput
 	InstrumentVoices      *NumberInput
 	TrackVoices           *NumberInput
 	InstrumentNameEditor  *widget.Editor
@@ -325,6 +326,11 @@ func (t *Tracker) DeleteInstrument() {
 func (t *Tracker) SetCurrentNote(iv byte) {
 	t.SaveUndo()
 	t.song.Tracks[t.Cursor.Track].Patterns[t.song.Tracks[t.Cursor.Track].Sequence[t.Cursor.Pattern]][t.Cursor.Row] = iv
+	if !t.Playing || !t.NoteTracking {
+		t.Cursor.Row += t.Step.Value
+		t.ClampPositions()
+		t.Unselect()
+	}
 }
 
 func (t *Tracker) SetCurrentPattern(pat byte) {
@@ -338,6 +344,11 @@ func (t *Tracker) SetCurrentPattern(pat byte) {
 		t.song.Tracks[t.Cursor.Track].Patterns = append(t.song.Tracks[t.Cursor.Track].Patterns, tail...)
 	}
 	t.song.Tracks[t.Cursor.Track].Sequence[t.Cursor.Pattern] = pat
+	if t.Step.Value > 0 && (!t.Playing || !t.NoteTracking) {
+		t.Cursor.Pattern++
+		t.ClampPositions()
+		t.Unselect()
+	}
 }
 
 func (t *Tracker) SetSongLength(value int) {
@@ -627,6 +638,11 @@ func (t *Tracker) DeleteSelection() {
 			t.song.Tracks[c].Patterns[p][s.Row] = 1
 		}
 	}
+	if (!t.Playing || !t.NoteTracking) && t.Step.Value > 0 && r1 == r2 {
+		t.Cursor.Row += t.Step.Value
+		t.ClampPositions()
+		t.Unselect()
+	}
 }
 
 func (t *Tracker) Unselect() {
@@ -642,6 +658,7 @@ func New(audioContext sointu.AudioContext, synthService sointu.SynthService) *Tr
 		SongLength:            new(NumberInput),
 		RowsPerPattern:        new(NumberInput),
 		RowsPerBeat:           new(NumberInput),
+		Step:                  new(NumberInput),
 		InstrumentVoices:      new(NumberInput),
 		TrackVoices:           new(NumberInput),
 		InstrumentNameEditor:  &widget.Editor{SingleLine: true, Submit: true, Alignment: text.Middle},