This is implemented entirely inside LibWeb, there is no GUI::CheckBox
widget instantiated, unlike other input types. All input types should
be moved to this new style of implementation.
Note that these aren't full implementations of the bindings. This
mostly implements the low hanging fruit (namely, basic reflections)
There are some attributes that should be USVString instead of
DOMString. However, USVString is a slightly different definition
of DOMString, so it should suffice for now.
