[Approved] Plasma 6 Proposal: Use "Clicking in scrollbar track: Scrolls to the clicked location" by default

Reasons to do this:
- This makes scrolling large distances faster, which can also be better for people with repetitive strain injuries.
- QQC2 does this by default, so it requires no effort to support. We have to put in extra effort to support the other way.
Reasons not to do this:
- It's different.
- It does not work with Tcl/Tk (toolkit used by `git gui`).
Other things to know:
- The ability to switch comes from `QStyle::SH_ScrollBar_LeftClickAbsolutePosition` (proposed) and `QStyle::SH_ScrollBar_MiddleClickAbsolutePosition` (current).
- It works with Firefox, Chromium/Electron, GTK and wxWidget apps.
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