Start to implement permission checks for tree editor and item editor.

Add a new permission "edit_foo" which means "can modify foo content base meta data". This is different than change_foo in that change will allow/forbid accessing the tree editor, but edit_foo will allow/forbid the item editor.
Also filter out available content types according to the user's add permission.

There probably are lots of holes in the permission checks still, so caveat emptor.
Merge branch 'next' into tabbed_options