Widget groups. Useful for when you need to swap out many
instances of the same widget type in a specific location in an
interface. A group has a currently active widget which can be
setCurrentGroupEntry. Add a widget to a group with
addToGroup returns an action which can be used to
set the specified widget as the group's active widget.
Add a widget to a group. Returns an action which, when evaluated, will update the group state so that its currently-active widget is the one passed to this function.