[html-formfu] new 'force_default' attribute for elements

[Mario Minati]

Today I had to implement a disabled form field, which should keep it's value 
no matter what the user sends in the form data.

So I created the 'force_default' attribute which overwrites the value in param 
with the default value.

The changes and documentation is in Elements::_Field.

BTW: I changed the formatting of a complex ??:?: to understand it.

Greets,
Mario