Blender Git Loki

BKE_object: add util to count number of scenes using an object.

Issue is, ob->id.us is not relevant anymore here, since several
collection might be referencing it inside of a same scene, that is still
only one usage from user perspective...

Note that for now we are just counting scenes instantiating an object,
time will say wether we need more refined/complete check (as a reminder,
most [all?] other Object usages are *not* refcounting ones).