mw.Title: Implement #makeTitle for titles with predefined namespace

mw.Title: Implement #makeTitle for titles with predefined namespace

Unlike #newFromText or the constructor, this function doesn't allow
the given `namespace` to be overridden by a namespace prefix in
`title`.

Name was chosen to match the PHP function Title::makeTitle().
(There's no need for a #makeTitleSafe variant, because #makeTitle is
already "safe" and checks title validity.)

Change-Id: Ib78dbcb0910bf1458b842b4d9ece6d204761b202