Multi-part post params not present in request object

The params present in a multi-part post do not get set into the request object (in this case the MuliPartRequestWrapper). As a result, code that wants to make use of these params needs to do its own multi-part parsing to extract the params.

Since we already have code that handles this for file uploads, I propose we move that code from UploadAction up into XWikiAction so that the params can be inserted into the request object iff we are processing a multi-part post.

I've already written the code to do this, just need to clean it up some and get some feedback as to the ordering of calls in XWikiAction - Ideally, this should happen before anything else so that any core code that needs these params can have acces to them. Unfortunately, FileUploadPlugin makes use of XWikiContext and XWiki objects, so these need to be constructed before making the multi-part params available.