I came across with the problem of getting checkbox value.
The current CheckBox implementation implies that only boolean values are
expected, coupled with "checked" attribute.
But I wanted to get the value other than boolean, like the primary key of
the database table. I expected the html tag as bellow:
<input name="optionalanswer" type="checkbox" id="form_optionalanswer"
I think checkbox value should not go together with the "checked" toggle status,
so it would not be necessary to override getValue(), setValue() etc. methods.