Fix extensions to work with category sort changes

Fix extensions to work with category sort changes

Three different extensions were copy-pasting all of
CategoryPage::closeShowCategory() just to change which CategoryViewer
descendant was used.  I changed the base class to use a member variable
for the CategoryViewer class, so this reduces code duplication too.  I
only tested this on CategoryTree; MetavidWiki is certainly still broken,
from the looks of it (assumes $this->from is a string), but
PeopleCategories might be okay.  Hard to avoid breaking extensions with
this sort of schema/index change.