Introduce readonly XWikiContext provider

In some case you don't care about getting a valid XWikiContext for sure, you just want the current one or null if none is set.

In some low level components like User/Wiki/SpaceComponentManager generating a new valid XWikiContext can even create infinite loop because some stuff in the XWikiContext generator require to load some components which end up in User/Wiki/SpaceComponentManager again, etc.