Blender Git Loki

I18n: dirty workaround for PITA ghost classes remaining after unregister.

Spent again a whole day trying to understand what happens here, with no
luck. For some reasons, OperatorProperties children of unregistered
operator classes remain accessible, with their `bl_rna` member, although
that one is 100% invalid (freed memory, crashes with ASAN builds).

Funny thing is, I cannot reproduce that situation when disabling the
add-on from the py console of a Blender-with-UI.

Note: issue revealed by X3D add-on, which is still enabled in factory
settings, while not being officially supported any more, this has to be
fixed in a separate commit.