Provide a way to add XML comment about unsupported XRC control.

[KeyWorksRW]

Description:

Currently, generators for controls not supported by XRC can insert a comment about the control not being supported. However, if the generator returns xrc_not_supported then the comment never gets added to the XML file. On the flip side, if a comment is added about the control being unsupported, and xrc_updated is returned, then an object with no class name is created. There needs to be a way to add a comment without adding an object.