mw.Title: Add method to get title relative to an arbitrary namespace

I'm not sure where this kind of thing could be used outside of
dealing with templates/transclusion. Making it generic anyway.

Bug: 67448
Change-Id: Ie554adefec43997d362b5d7b45c30403912743b5
(cherry picked from commit 63e6ebb8fac41e8d57aa7f9677d4419155b01fc1)