ScrollView doesn't have pressDelay

I'm not sure if it should have it, because it's a kludge to implement, and I don't want to see us write yet another copy of that code, creating a synthetic event, another timer and all that. But users often use Flickable.pressDelay to work around grab problems. So I guess if they want it, they need to wrap an existing Flickable with a ScrollView, which is yet another thing that doesn't fit very well with that proposed story-line that Flickable is too low-level for most UIs and everyone should use ScrollView instead.

One decent way to implement it is if we are sure ScrollView will always depend on Flickable (either an existing one or a new one), it can have the property and the accessors can "pass through" the value to whichever Flickable.

Another way is to move the delayed-press functionality to QQuickDeliveryAgent and make it reusable. (Or maybe even to QtGui?) Probably that's eventually needed anyway.