res:dock:: fix menu position problem on multi-screen.

menu will get wrong position.

reason:
the item's menu position is pageXY, and MouseEvent.clientX/MouseEvent.clientY
are not used, so that this problem is missed on last time.

solution:
using MouseEvent.screenX - MouseEvent.pageX and MouseEvent.screenY -
MouseEvent.pageY to get screen offset.

Change-Id: I665926bea2e9721b0a97d3e9a6847262eac6abca