Hi,

a few days ago, it was decided to create a new function BKE_object_relational_superset()
which shall become a standard function for finding export sets of objects.
This function is first tested with the Collada exporter.

this patch provides

- an implementation of BKE_object_relational_superset()
- a rewrite of the Collada exporter to use this function

I have tested the exporter briefly.
It works for all my test cases as expected.

this is a request for reviewing the patch
and get feedback.

I believe that when the patch is accepted, then i would
separate it into 2 subsequent parts:

1.) the implementation of BKE_object_relational_superset()
2.) the Collada rewrite

thanks for your feedback!