TTMSFNCButton extended to support 'default' and 'cancel' behaviours

On a normal TButton, there are properties for Default (pressing enter) and Cancel (pressing Escape). These are quite common behaviours on a form.

Currently, TTMSFNCButton only supports basic button behaviour. Although it should be easy enough to add, it may be nice to consider support as these are fairly common use cases which need to be accommodated, rather than a case by case basis individually for modal forms.

I appreciate Web Core may need special consideration, but the behaviour can still be useful if modal forms are used.