taskswitcher: polish task scrub mode

task scrub mode needs some polishing:

• better visual indication that it is active (eg: list of window icons below window previews?)
• the activation threshold is a bit too small on small devices making it hard to consistently activate
• no way to go "infinitely far back" -> maybe when reaching the "end of the screen" start a timer after which it goes back one task per second?
• revise activation logic -> eg: right now you can still flick if fast enough even if all other conditions work for scrubbing, but the flick needs to be "fast". since we don't have a reliable way of getting screen DPI at least on some devices that velocity limit could make the interaction feel weird and inconsistent.
• add flick upwards gesture to dismiss scrub mode?