Replace the boolean 'started' member by an enumeration which indicates
the startup phase. While in that phase, do not cancel when pressed
points are detected for fewer than 3 touch points.

Task-number: QTBUG-15768
Change-Id: Ic57b19e3002392fb632f551f615d80ada9831d34
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>