Replace homemade ScrollHelper with standard ScrollViews to hold view items

We created the ScrollHelper component to fix scrolling with touchpads,
which is quite terrible out of the box with QML's default flickables.

However this solution had some drawbacks:

- It broke touch scrolling
- It broke fast scrolling by holding down the shift key while scrolling
- It essentially re-wrote scroll handling locally, which is fragile
- It required a bunch of extra code to handle scrollbar visibility which
  introduced various minor visual bugs in a few places

In addition, this solution was not necessary since we can get the
same result by putting views inside ScrollView components, which is a
100% supported and standard paradigm. Doing so preserves the nice
touchpad scrolling we already have but fixes all of the above items
automatically.

BUG: 417859
BUG: 427967
FIXED-IN: 20.12