Zoom at where mouse is at

[Angus Yeh]

(Suggested by John and several others) The zoom mechanism needs to be improved to something like zoom at where mouse is at.

[Angus Yeh]

I should make it behave like Google Maps: After zoomed-in, where mouse was originally pointed at is still where mouse is pointing at.

[Angus Yeh]

Fix Issue #20, now mouse scroll zoom keeps the scene position under mouse.

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