Add ability to create S-Groups with other S-Groups inside

If user creates new S-Group over existing S-Group with the same set of atoms following problems appear(see https://github.com/epam/ketcher/issues/775):
1. Brackets of both groups have the same coordinates, so we can see only one set
2. Labels are overlaped
3. It's impossible to see which group is inner group

Expected result:
1. Position of brackets and labels differ for each S-group
2. User can see which group is inner group