Allow updating members of existing groups in the user output filter stream

Currently, it is possible to create new groups from filters. Updating an existing group will actually completely replace the group.

It would be useful to be able to update groups and add (and remove?) users instead of replacing groups. One use case would be to add admin users to XWiki.XWikiAdminGroup from the confluence-xml module when importing data from a Confluence wiki (including users).

This could be done by adding a GroupFilter#addGroupMemberUser method, or to introduce a GroupUpdateFilter class.