Added a lock-free deferred queue for deletion. Now if ID icon
is requested to be freed from non-main thread, it will be added
to the deferred list. Actual deletion will happen later from main
thread.
Currently actual deletion only happens next time BKE_icon_id_delete()
is called, which might not be enough. But it's easy to enforce
deferred deletion.
Icons for preview images are not covered by deferred deletion yet.
Reviewers: mont29
Differential Revision: https://developer.blender.org/D3146
6617818c7a