refactor(tracker/gioui): rewrote Button(s) to bind to Model during layout

The old mechanism made it difficult to follow exactly what happens
when a button was clicked, because the Action/Bool that gets
executed / toggled was declared ages ago, in the constructor. In the
new mechanism, the Action / Bool is bound to the button at the last
minute, right before Layout. ActionButton, ToggleButton,
ActionIconButton and ToggleIconButton were done to avoid heap
escapes: if the corresponding functions woudl've returned
layout.Widget, a heap allocation would've been needed.

tracker/gioui/tracker.go

@@ -210,7 +210,7 @@ func (t *Tracker) Layout(gtx layout.Context, w *app.Window) {
 	paint.Fill(gtx.Ops, t.Theme.Material.Bg)
 	event.Op(gtx.Ops, t) // area for capturing scroll events
 
-	if t.InstrumentEditor.enlargeBtn.Bool.Value() {
+	if t.InstrEnlarged().Value() {
 		t.layoutTop(gtx)
 	} else {
 		t.VerticalSplit.Layout(gtx,