Simple Modal Help Protocol
--------------------------

Windows which support interactive help should set a
_GPE_INTERACTIVE_HELP property on themselves. 

When a mouse click happens in help mode, the help application will
send a ClientMessage event to the appropriate window, with type
_GPE_INTERACTIVE_HELP, empty event mask, and payload:

l[0]	a Window
l[1]	x coordinate of mouse click relative to destination window
l[2]	y coordinate of mouse click relative to destination window
l[3]	an Atom

The receiving program should respond to this message by setting the
desired help text as the contents of a STRING property named by the
Window and Atom specified in the clientmessage.