Use mouse wheel for selecting piece in Edit Position mode (XB)

Use mouse wheel for selecting piece in Edit Position mode (XB)

Rather than calling ForwardEvent() and BackwardEvent() directly from the
mouse-wheel handler, we now call a backend routine Wheel(), which gets
the pointer coordinates passed to it (which we must remember during
mouse move events). This then calls the Forward/Backward event, except
in EditPosition mode, where it changes the piece in the square below the
pointer.