Don't look for catalogs in AddCatalogLookupPathPrefix() path directly.

Don't look for catalogs in AddCatalogLookupPathPrefix() path directly.

Previously, the prefix directory itself for searched for catalogs, in
addition to prefix/lang/LC_MESSAGES and prefix/lang.  This never made
much sense, because only one catalog could be in such place, instead of
multiple catalogs for more languages.  It also prevented successful
identification of catalog's language and didn't work with
GetAvailableLanguages().

Remove this misfeature and update the documentation accordingly.  Update
OS X-specific code so that it continues to work (it previously depended
on this behavior).  As a side effect, *.lproj directories are now
recognized in all search prefixes, not just in Resources.

See #12498, #15253.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74441 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775