[REG 5.14] Touchscreen scrolling isn't momentum-based anymore

Similarly to QTBUG-71800 (which was fixed in https://codereview.qt-project.org/245571), touchscreen scrolling stops abruptly when lifting the finger.

This can be reproduced without a touchscreen in simplebrowser: Open the inspector, go to the three dots menu > more tools > sensors, scroll to the bottom and select "force enabled" for touch, then reload.

With Qt 5.13, the page will continue scrolling when click-dragging it, while with Qt 5.14, it does not.