ComboBox: open the popup on release on touch devices
ComboBox: open the popup on release on touch devices

This is the common behavior on touch devices, and avoids such problems
as opening the popup while trying to flick a ScrollView that contains
a ComboBox.

Change-Id: I9207bc2c00524fe4cd5a673f4fe505f6db825838
Reviewed-by: Caroline Chao <cb6a0059f4e516e853a34cb558c25ca5a9bcf4ab@digia.com>

mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols