Issue title: [core] Group item can be added into itself
 Issue body: A Group item can be added into itself. Is this supposed to be possible?

demo.items
```
Group gHISTORY "Persistence" (gHISTORY)
```

console output
```
openhab> items list gHISTORY
gHISTORY (Type=GroupItem, Members=0, State=NULL, Label=Persistence, Category=null, Groups=[gHISTORY])
```

 Comments: 
 Comment 0: More generally, you can define a loop in the tree. That is certainly not expected.
 Comment 1: Why isn't it expected? To my knowledge the framework can cope with cycles quite well. So this boils down to the question of whether the framework needs to patronize the user in this case "just for fun". Isn't this also kind of a "s**t in, s**t out"-situation?
{'source': 'GithubIssues', 'repo_name_with_owner': 'eclipse/smarthome', 'issue_id': 326633223, 'repo_stars': 952.0}