Add tap-to-scroll to the scroll bars

[dweymouth]

Description:

Tapping above, below, left, or right of a scroll bar will scroll the content up, down, left, or right by a (nearly) full page, respectively. When the scroll bar is large, a new background rectangle is drawn behind it to indicate the taps will go to the scroller and not the underlying content.

Fixes #4922

out.mp4

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.