Revamp GTK's session close handling using toplevel callbacks.

Revamp GTK's session close handling using toplevel callbacks.

Instead of having a special GTK idle function for dealing with session
closing, I now use the new top-level callback mechanism which is
slightly simpler for calling a one-off function.

Also in this commit, I've arranged for connection_fatal to queue a
call to the same session close function after displaying the message
box, with the effect that now all the same processing takes place no
matter whether the session closes cleanly or uncleanly - e.g. the SSH
specials submenu is cleaned out, as it should be.

git-svn-id: http://svn.tartarus.org/sgt/putty@10022 cda61777-01e9-0310-a592-d414129be87e