Velocities reported by Flickable in onFlickStarted can be 0

Velocities reported by Flickable in onFlickStarted can be 0

Change 55cfbdb7c64068ae68f7baaceb8acfb96cb0c07e manually applied
from Qt 4.7

Ensure the smoothed velocity is set at the start of the flick.  Ensure
that the smoothed velocity animation isn't restarted unless there is
new valid data.

Change-Id: Ia77249be9980aba268a1bfa0ea3f69c49fa09e5e
Reviewed-by: Bea Lam
Reviewed-on: http://codereview.qt-project.org/4712
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>