support components

[mathildedd]

I am quite unhappy about having to redeclare the subfields as class elements, I think that they should already be class elements, but currently they are arrays.

[Berdir]

As discussed.

make the component list on group public, reference it from there, fix the property definitions method too and then maybe we can work with that.

also in the new and existing code, check if that field type is defined:

(if !isset()) {
throw new \Exception('Unknown field type $foo');
}