U4-6911 - Content type folders

That's because the feature is incomplete and still requires a bunch of work ;)

Looking it this now, I think we should just switch of the list view feature, so its consistent with partial views folders.

I have finished this now and it's all working, there's also new unit tests. There's quite a lot of code in there so please ask questions if any of it doesn't make sense.

To Test:

• Run the unit tests
• In all three of these trees: content types, media types and data types: ** Create folders and sub folders
  ** Delete folders and sub folders ** Move an item to a folder and sub folder ** Move an item from a folder to another folder ** Move an item back to the root
• Ensures any content and media items associated with any moved content types, media types and data types behave the same
• Install a package that also installs custom data types and content types (i.e. Articulate is one)

Courier does not support this yet, so don't test that it won't work.

PR is here for review: https://github.com/umbraco/Umbraco-CMS/pull/900

Q: is there a way to (1) rename a container and (2) move a container?

Nope... not going to have that functionality, at least for this release. All other folders (i.e. file system folders) cannot be directly renamed either.

Merged the PR.