mw.Map: add ability to map over an existing object other than 'window'

I don't love the fact that mw.Map features so prominently in mediawiki.js, but
since it's there to stay it might as well be useful for a wider range of use
cases. This patch makes it possible to pass an existing value-bearing object to
be mapped over to the Map constructor.

The effect is the same as constructing the object and then overriding its
'values' attribute. But doing that feels like you're sneaking around the back
to hack the API. Making it part of the official API makes it OK to do.

Change-Id: Id9f8d9569de8889fc3ffa24a6e7f4afca1aeabee
Merge "mw.Map: add ability to map over an existing object other than 'window'"