Merge down and merge groups with multiple objects

[MathemanFlo]

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.