Port change 67ed1849 from Qt 4.7

Port change 67ed1849 from Qt 4.7

Flickable could flick in wrong direction if given too few touch samples

If we got <= QML_FLICK_DISCARDSAMPLES the previous velocity was not
cleared, so the view would continue flicking with the previous
velocity, and therefore the previous direction.

Task-number: QT-4903

Change-Id: Ie4c2a3829026c0bf7d94cf5b60c0b7ff9833f725
Reviewed-on: http://codereview.qt-project.org/4618
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>