add Class registration to override type-based Field class selection when field type is default

This patch allows to register a cell class for a generic class (mimicking registerCellClass:forFieldType:):

- (void)registerCellClass:(Class)cellClass forClassName:(NSString*)fieldClassName;

so that the registered cell class is used for attributes of the given class. The fieldType has priority over the className, i.e., the registered class is used only when the fieldType is FXFormFieldTypeDefault.

In short, this is another mechanism to override the default cell/property relationship; instead of defining a method xxxxxField returning the proper information, you register a cell class for a specific form class.