mediawiki.api: Don't apply file extension stripping to category names

`mw.Title#getNameText()` strips anything following a dot (.), assuming
it's a file name (which doesn't make sense for category names).

C.f. https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Title-method-getMainText

This causes some trouble in UploadWizard's category-suggestion system.

This bug has been there since the very first version (cb0cf72eba9ff)
which, ironically, was specifically written for UploadWizard.

Bug: 64764
Change-Id: I2925e1050215272cdf03c8c6b1de6ba79348f6a5