New code loops much less, does re-open & close .blend files for each data type,
and is much much more flexible - it is also ready for id-remap & co work being done in branches.
Main idea is to store libraries & datablocks to link in a dedicated struct, in a way
that avoids too much looping, and also allows to search for a single datablock in several libraries.
Here again, no change is expected in current behavior of link/append tool, please report
if anything goes different!
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