)
Call DVeop entry point with:
	ES:DI -> callback address or 0000h:0000h to remove callback
Return: AX = status
	    0000h failed (callback table full or attempted to remove non-
		  existent callback)
	    0001h successful
		ES:DI -> chaining address
	BX,CX,DX destroyed
Notes:	the callback function is called with a simulated interrupt when the
	  DESQview window containing it is closed; it should perform all
	  necessary cleanup and then perform a FAR jump to the chaining address
	  or an IRET if the chaining address is 0000h:0000h
	if the program wishes to remove itself before the window is closed, it
	  should call the DVeop entry point with the previously returned
	  chaining address and ignore the returned chaining address.