See T95597

In particular, this commit adds support for override libraries on
objects. In the past, if you made an override, Edit Linked Library lost
the reference to the original library. That's fixed now.

Also, in linked Node Groups, this commit also adds support for linked
Sverchok group nodes.

If you have a relative path (e.g. //../objects/object.blend) and you use
bpy.path.abspath, it leaves the relative part (the double dots) as part
of the path (e.g. ~/some_project/scenes/../objects/object.blend). While
Windows is perfectly happy to open files with this kind of path, it
seems to get confused about paths that are relative to that. The fix is
to wrap the bpy.path.abspath function with os.path.abspath and
everything appears to remain sane.

Also introduced a small fix for the new linked node groups feature; in
2.93, the Item tab in the sidebar has been renamed to Node.

Also re-enabled editing from a separate instance of Blender for things
in the 3D Viewport as well as in any of the Node Editors.

Differential Revision: https://developer.blender.org/D4761

Differential Revision: https://developer.blender.org/D4351

Differential Revision: https://developer.blender.org/D4584

Differential Revision: https://developer.blender.org/D4070

This follows naming convention agreed on in T56648.

Just abort when trying to (auto)save current file and it does not exists on disk yet.

The Edit Linked Library panel now only appears in the Relations tab

[[Split portion of a mixed commit.]]