Add a constant to the default width and height of the forms

When trying to auto-calculate the width and heigth of a form based
in AbstractForm, the scroll-bars and borders add some pixels to the
size that must be taken into account.
Don't check the enability of a widget when filling values

When the form is filling the initial values, widgets like cb fire
events that can be processed by the EnableComponentsBasedOnWidget
listener, and this can cause errors, if the slave widget has not
been filled previously. So, following the patterns in
ValidationHandler the isFillingValues method is used as a flag to
avoid check the enability