use the total length of zoomDelta, not just the y component, for multi-touch zooming

this means the 'pinch' gesture can be oriented any way the user desires (in fact, pinching along the x axis is the more natural variant) and still yield the desired result
fix typo in description of onMouseMove()
Merge pull request #64 from T045T/devel

multi-touch zooming: consider movement along X and Y axis