Too many built-in content types

[tusmester]

THIS IS A DISCUSSION ISSUE

There are too many content types visible in the default content type editor that may confuse new users.

1. collect content types that can be removed from the system altogether
2. find a solution for hiding system content types
   • what does system type mean? Is this a flag on a type?
   • what types should be system?
   • how do we hide/display them on the UI? A simple switch on the Content type grid?
3. multiple categories instead of a system switch, filter buttons on the UI?
   • system types
   • default types
   • my custom types
4. bring back the content type tree?