Allow (optional) title normalization in wbgetentities

Allow normalizing the client page name we try to find
an entity for. For performance reasons we only do this
if the user asked for it and only for one title!

This also introduces the new API helper class
\Wikibase\Api\ItemByTitleHelper.

Change-Id: I1d010e5d5e07209f23c93636163f722d0d247a29
Merge "Allow (optional) title normalization in wbgetentities"