QPanGestureRecognizer: Make the number of touch points a parameter.

Prepare for determining the suitable number of touch points
from the device type. For now, 2 points are used as
before, which can be overridden by setting the environment
variable QT_PAN_TOUCHPOINTS. Add member variable
to QPanGesturePrivate which is set on gesture creation and later
used for comparison.

Task-number: QTBUG-40461
Change-Id: I6d9e35ca752375bc6a54435482ca0925195b8142
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>