Allow to construct better layouts with forms

* Add a 'form_columns' property to specify the quantity of columns on a form
  for fields. Default is 1 to keep compatibility.

* Add an EmptyField field to be used as a marker for empty spaces.

* Add a colspan property for fields so on a form with more than 1 column, one
  field could expand and use 2 columns (just like html).

Change-Id: I90661f65fb141a2ae099343c2f04d61c83291019