pinch and drag threshold signatures added; minor bug fixes

This release adds 4 new signatures, getter/setters for pinch and drag start thresholds (pixel values that must be met by user interaction before UI operations begin). The last commit by @onder7973 had these as constant values - this commit updates them to private members with getter/setters. Also included is a patch to end fling operations when a drag operation starts (thanks @redred77), and a patch to fix a bug with canvas.quickReject and hardware accelerated views (thanks @onder7973).

This release should be production-ready (build status passing), but could use further testing.