modified_in_session, to support flagging new and session objects.

  - Support editing content-type,
  - Separate changing properties and data.

Also changed heading TDs to THs.

Modified editing interface to:
  - Support editing content-type,
  - Separate changing properties and data.

compatible with the new addObject protocol.

<some damn instance as 123ab34c>

exception when called and the exception type and value are not
AttributeError and '__call__'.

rendering.