Added ability to support any property object type

In the same way the user can register a cell class for a field type he can now register a field type for a property class.

This way I am able to register "UIColor" to give the field type "color". I then register the field type "color" with my own FXFormBaseCell derivative "FXFormColorChooserCell". And now FXForms will automatically be able to support showing and editing UIColor attributes without any field specific stuff.

The translation from class to field type searches upwards in the controller hierarchy so it is sufficient to do the register at the top level - this kind of functionality is really something you are using to minimize the configuration of specific fields.