Firm-up the over bound Flickable behavior

When the end of a list is reached or you flick past the bounds the
amount of tension is too weak and items fly far to far past their bounds
or can be pulled far to far down from the edge

Reduce the default maximum overshoot.  Ensure that the amount we have
already flicked past the bound is considered in the overshoot
calculation.

Task-number: QTBUG-25427
Change-Id: Ia102b9995f944beebf6b4efc6ee35c6e9fe56d79
Reviewed-by: Bea Lam <bea.lam@nokia.com>