Blender Git Loki

LibLink: New Append code.

This commit fully refactors the way linked IDs are made local when
appended.

Previously, `BKE_library_make_local` was (ab)used for this task, but it
was missing some contextual data and doing complex processing to try to
work around this, with limited sucess. Further more, it was nearly
impossibe to extend (e.g. to get new append behaviors necessary for the
asset project).

The new code is a dedicated append step in WM linking process.

NOTE: BPY API (`libray.load()` context manager) uses its own code here,
which still relies on `BKE_library_make_local` for appending.
Unfortunately, merging those two different code paths is not trivial so
for now this API will remain unchanged.

Fix T55629: Append already linked Data is impossible.