The HTML <label> element is special in that it may be associated with
some other <input> element. When the label element is clicked, the input
element should be activated.
To achieve this, a LableableNode base class is introduced to provide an
interface for "labelable" elements to handle mouse events on their
associated labels. This not only allows clicking the label to activate
the input, but dragging the mouse from the label to the input (and vice-
versa) while the mouse is clicked will also active the label.
As of this commit, this infrastructure is not hooked up to any elements.
