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.
