(xterm-mouse-translate): Only generate down event when bound.
Cleanup. Support for mode line and vertical bar dragging, and
menu-bar events.
(xterm-mouse-x): New variable.
(xterm-mouse-y): New variable.
(mouse-position): Advice to use above variables.
(xterm-mouse-event): Support menu-bar. Set xterm-mouse-x and xterm-mouse-y.