Make style/class informations part of the XDOM specifications

The issue is that currently we are producing in many macros as well as the WYSIWYG datas which are meaningful only for XHTML and not part of the XDOM specifications. That means that no renderer except XHTML renderer can understand them officially.

And the thing is that theses are really not XHTML specifics informations since we are talking about colors etc.

IMO we should at least make part of the XDOM spec a style syntax with well know values and possible custom values for XHTML only fancy thing you can put in "style" attribute. This solution could be a not expensive solution (since it does not change anything and just make official the syntax of some custom parameters) waiting for maybe some more typed thing in FormatBlock or something at some point.

That would also fix things like merging nested style in the renderers that does not support nested styles.