Overridden. If this Form is modal, and the user clicked on OK (or accepted it by typing Enter), this calls ModifyObjects to commit any changes selected by the user. If this Form is modeless, this just Hides the Form.
Overridden.
Determine whether the OK and Cancel buttons are visible,
according to whether this Form is modal, and if so, position them
appropriately at the bottom right corner of the Form.