Avoid injecting RenderingCache and DocumentDisplayer in all instance of XWikiDocuments

Direct injections of components used only partially cause them to be instantiated too early, and for many tests, even before the whole environment is ready. Please avoid injecting stuffs in private old code objects, without a real need.
In this particular case, we have those in XWikiDocument because of the getRenderedContent() which is itself misplaced.

I am moving the lookup into that function, using the private variables as a cache.