add exwm--client-message-functions

* exwm.el (exwm--client-message-functions):
Alist for dispatching client messages to handlers.
(exwm-init): set exwm--client-message-functions once exwmh support enabled.
(exwm--on-ClientMessage): delegate via exwm--client-message-functions,
decompose body into separate handlers.

See: ch11ng/exwm#931