Implement a continuous version of 'Quick Return'

This is a 'continuous' version of QuickReturn. QuickReturn
itself is rather discrete - you can never actually be stuck with
the bar in an 'inbetween' state. This is a much simpler version
that just continuously slides it around. The discrete version
requires some statekeeping mechanics.

Change-Id: Ib9c5d33e9b43f6199d767a8d20567623b89a9dd1