Various edits preparing addons' translations tools (not everything yet functionnal/tested, though).

Also workaround a nasty bug, where unregistered py classes remain listed in relevant __subclasses__() calls, which would lead to crash with python addons i18n tools (main translation was not affected, as messages extracting tools are executed in a brand new "factory startup" Blender ;) ).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55920 954f8c5b-7b00-dc11-b283-0030488c597c