Introduce an author setter helper in XWikiDocument for the most common use case

[tmortagne]

@nikpetrenko

Hello, will this simplify cases where another user replaces a document’s author, but the content’s author will still be the old author?

This case will be covered by this API too, yes. The content author is automatically set to the provided author but only if the content dirty flag is true (i.e. if the content was modified).