Blender Git Loki

WM linking code rework.

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!