Blender Git Loki

WindowManager: Keep track or the source library when appending.

Keep track of the source library allowing other parts of the code to
to make better decisions. This is needed to localize external files.

In this case the file paths are updated when `making local`.
But we should decide based on the source library if we want to
copy the file relative to the new blend file. See D12423.

Reviewed By: mont29, Severin

Differential Revision: https://developer.blender.org/D12765