M5 Cleanup: add default folder to /content when creating a new documenttype

After having created a new documenttype by copying an existing one and altering it, I can't add this documenttype to a folder under /content.
Adding it using the console is pretty simple, but cumbersome.

Possible solutions:
1. automatically add a folder with the same name as the documenttype under /content, and assign it the hippo:prototype of the newly create documenttype
2. add "pick a documenttype" dropdown to the "Add folder" dialog and let the user input both the name of the folder and the prototype to use for that folder
3. ...?

Solution number 2 has my preference, but would mean that users should also be able to create folders under /content (which is now prohibited)