handle “unused_objects” after template change

• display unused objects in a “virtual” container “unused_objects” in the content tab of the page detail
• set container name to null if a content object is moved into this container
• change style of the container to help discriminate it's different purpose

additional advantage to a improved transparency
• complex module configurations can be temporarily removed by moving them to this container

optional drawback
• it might be to complex for normal users?

bug to be fixed (jm doesn't know how to) thought it looks like it works properly
• when moving the objects to this new container "_this.filled_frontend_modules[target.attr....] is undefined"
  (I guess this concerns only the position/order of objects?)
  see default.page_type.js.tmpl line 152
change visibility of edit button

• for template and page_type
• if there are no content objects configured for the current page, the page_type/template can be changed without seeing the content
  – if the page is new and it's page_type "default"
  – if the page_type is not of type "default" (unless there are dangling old content objects)