Merge down and merge groups with multiple objects

Currently merging down a layer or merging a group sets the object of the lowest layer as the new object. I'm not completely sure if this is the best behavior. In my opinion there are two cases:

1. All objects are the same, thus the merged layer should be assigned to this object.
2. The materials are mixed. I think it would be a meaningful behavior to change the object to Shared in this case or the disallow merging groups with different objects.

No files, tests, or entry points are named. First locate the layer and group merge behavior, then clarify the intended rule for same-object and mixed-object materials; done means the agreed behavior is implemented and verified for both cases.

Written by the indexing model from the issue text.