[circle] chat fields

[Thomas Voegtlin]

I modified the implementation of chat fields in chatgtk.py
(oh, and I also did some cleaning, moving gtk stuff from
chat.py into chatgtk.py)
The array chat.fields has been replaced by a double linked list.
this avoids the use of expensive operarions such as "if x in fields",
or fields.index(x). Inserting and deleting fields are now O(1), and
showing text is also improved.

So the chat window should be much faster now, especially if
you use the /find command to display long lists of files.

Since this is a big change, however, there might still be bugs
remaining. For example, some of the CVS versions of yesterday
were fairly broken. I think it is all fixed now, but please apologize
if you still find bugs.

Thomas