Change I/O signals to pass a unique window ID

Change the 'char-input', 'line-input', and 'text-buffer-output'
signals to pass a unique window ID string as well as the
window rock value. If a client program wants to keep track of
input and output in all windows, then the window rock will
not do by itself - all open windows might have rock 0, for
example. The string ID is guaranteed to be unique for each
window, though its contents are implementation-defined.