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/theme.go

@@ -19,6 +19,12 @@ type Theme struct {
 		Disabled ButtonStyle
 		Menu     ButtonStyle
 	}
+	IconButton struct {
+		Enabled  IconButtonStyle
+		Disabled IconButtonStyle
+		Emphasis IconButtonStyle
+		Error    IconButtonStyle
+	}
 	Oscilloscope  OscilloscopeStyle
 	NumericUpDown NumericUpDownStyle
 	DialogTitle   LabelStyle