This enables, for example, clicking on the check box, dragging the mouse
over to the label, releasing the mouse to act as a click on the check
box.
This was implemented for labels / labelable nodes with the "for"
attribute already. This implements the same for labelable nodes that are
inside the label.
After dispatching a "change" event due to the checked state being
modified, we may have been removed from the layout tree.
Make LayoutCheckBox protect itself to prevent this from crashing.
Also, add a little test page for checkboxes. :^)