Blender Git Loki

Fix T47482: take II, restore 2.76 behavior regarding groups when deleting an object.

Also re-reported through IRC by Thomas Beck (@plasmasolutions), thanks.

Though it's not ideal in theory, we have quite poor handling of object datablock currently
from user PoV - before this commit, it was not easily possible to get fully rid of an object
anymore if you did not removed it from all its groups before deleting it.

So for now, restore 2.76 behavior (namely, unlink an object from avaerything in Blender
once it is no more used by any scene).

Better handling of all this is TODO for later (also related to much more heavy changes
done in id-remap branch regarding sanitizing our ID deletion process).