Enable smooth scrolling and zooming on platforms that supports this

On platforms that support smooth (2D) scrolling, swap the behavior
of scroll vs modifier-scroll events and make use of the high-resolution
scroll delta information. Also enable native zoom events on platforms
that support this for QT >= 5.2. (I.e. pinch zoom on Mac)
Merge pull request #451 from OskarLinde/smooth_scroll

Enable smooth scrolling and zooming on platforms that supports this