Avoid freeing the backend in notify_remote_exit(), since that's

called from within a backend function which will expect its own
backend pointer to still be valid on return. Instead, move all the
real functionality of notify_remote_exit() out into a GTK idle
function.

git-svn-id: svn://svn.tartarus.org/sgt/putty@8304 cda61777-01e9-0310-a592-d414129be87e