Add `PropTypes.dataStructure` Validator

This is an exact copy of the shape-validator but reflects the term data-structure wich
is added as Fusion prototype in Neos 4.2.

PropTypes.dataStructure({'foo': PropTypes.integer, 'bar': PropTypes.string}): Validate the keys of the given array validate with the assigned Validator, accepts null and ignores all other keys. The key validators have to define wether a single key is required.