GitLab

refs #44

Fix #39

There was a slight window in which the event callback was still
registered, but the mutex & condition_variable were destroyed

Delegate subitem creation to the parent item. This way we can access the
internals of the parent item upon creation, and the item is created in
place.
This will also ease up privatization of the Item type, since we don't
want it to be publicly available for creation

It is now up to the user to request thumbnails generation.
It allows lazy loading, and prevents us from potentially generating
thousands of thumbnails for nothing when indexing an entire network
drive.