(bug 7560) Separate Special:Categories name and categorybox header

(bug 7560) Separate Special:Categories name and categorybox header
* Created a new message, 'pagecategories', which is used instead of 'categories' in the categorylinks box under the articles.
* Duplicated 'categories' to 'pagecategories' in all the languages.
* Removed "{{PLURAL}}" from 'categories' (and not 'pagecategories'), as it's ignored when used as the title of Special:Categories, and was added for the categorybox which now uses 'pagecategories'. It makes no harm, but it's useless, as no parameter and "{{PLURAL}}" is used for 'categories' - only for 'pagecategories'.