* xt-mouse.el: Implement extended mouse coordinates.

* xt-mouse.el: Implement extended mouse coordinates.
(xterm-mouse-translate): Move code into xterm-mouse-translate-1.
(xterm-mouse-translate-extended, xterm-mouse-translate-1)
(xterm-mouse--read-event-sequence-1000)
(xterm-mouse--read-event-sequence-1006): New functions.  For old
mouse protocol, handle M-mouse-X events correctly.
(xterm-mouse-event): New arg specifying mouse protocol.
(turn-on-xterm-mouse-tracking-on-terminal)
(turn-off-xterm-mouse-tracking-on-terminal): Send DEC 1006
sequence to toggle extended coordinates on newer XTerms.  This
appears to be harmless on terminals which do not support this.