Touchscreen scrolling needs adjustments

GTK's touchscreen scrolling isn't exactly pleasurable to use. The most annoying issue with it is velocity calculation when the finger is released.

When the finger isn't moving, releasing it shouldn't trigger further scrolling. However, with GTK it's extremely easy to trigger either on devkit or the phone, and even on my laptop - which is something that doesn't happen in other apps (like Firefox, Chromium or Qt-based ones).