Refactor a bit the way we load the generator libraries: hide the hash with the Generator completely into the Document, so the Document have the complete control over them.
Add fillConfigDialog() and supportedMimeTypes() to the Document, so the Part can get all the info it needs with no need to own the generator structure.
This also loads all the generators when asking for their config widget, but that can be improved easily now; furthermore, all the generators used to load documents are kept.

svn path=/trunk/playground/graphics/okular/; revision=627909