[html-formfu] Reorganizing DependOn and MinMaxFields Constraints

[Mario Minati]

Hi Carl,

as the DependOn constraint is only a specialisation of the MinMaxFields 
constraint I would like to make it a subclass of it.

Although I think of changing the behaviour of MinMaxFields if no max value is 
given:
Currently I'm setting max to 1, I would like to set it to the number of 
elements in 'others'.

Greets,
Mario