Blender Git Loki

undoexp: much better handling of idname-based re-using old IDs.

This commit adds obvious usage of a proper mapping to find local IDs
in old Main based on their names.

It also adds handling of potential pointer collisions between old Main
realm and read .blend memfile realm. That last part is theoritical
currently though, testing it actually works won't be easy (pointer
collisions will happen, but they are not very likely to happen...).

We are still missing some way to prevent all that new code to be ran
over an undo step that contains ID renaming (ID creation/deletion should
be safe, but idname-based search cannot work whith renaming).