Allow customizing Flickable bounce back animation

Flickable bounce back animation feels a bit slow. AFAICS Currently the bounce animation follows a hardcoded 400ms animation duration (see fixUp variable in qquickflickable.cpp), which hasn't been touched for at least over a decade. Provide a way to customize bounce duration (and/or replace the default animation with e.g. custom easing curves) to allow platform interaction style customization without forking Qt or coming up with a custom flickable implementation.

The average duration of touch device animations have gradually been getting faster in the industry over the years (I guess in parallel to the overall speed of operation improvements). In general would be good to revisit all the default in-built animations with some experienced UI designer.