Blender Git Loki

Fix part II of T53977: Severe problem with multiple instances of a library (save and reload).

Once 'losing lib' issue is fixed (in previous commit), we have new issue
that this could lead to several copies of the same linked data-block in
.blend file. Which is not good. At all.

So had to add a GHash-based check in libraries reading code to ensure we
only load a same ID from a same lib once.