New text converter interface

[Fred Kiefer]

I just rewrote part of the text converter interface of
NSAttributedString. The new interface should be more powerful, as it
takes another options parameter and has the ability to return NSError
objects. It is also simpler as there is only one method left to
implement for each consumer or producer. And the whole text converter
concept is now fully extensible, any new text converter should be picked
up now and used by NSAttributedString. That way it should be fairly
simple to write a text converter for HTML or what ever other format you
want to support and ship it as a separate bundle that will be picked up
by gui at runtime.

If you already have a private text converter you will have to adjust the
interface. I already did that for the only text converters provided by
gui (RTF and RTFD).

Fred