Use velocity from touch events only when they are valid

Add the capability flags to the extended mouse events. Otherwise it is
not possible to tell if the velocity is valid.

While the original version is fine if velocity is guaranteed to be
available whenever QT_TRANSLATE_TOUCH_TO_MOUSE is set, some platform
and driver combinations, e.g.  the evdevtouch plugin that comes with
Qt, do not provide velocity data in touch events. The touch-only mode
of QML may be very useful in these cases too, we just need to fall
back to the built-in velocity calculation.

Change-Id: Iec5e7632a66380dc04c9435b09f5c173107bbe00
Reviewed-by: Martin Jones <martin.jones@nokia.com>