Add overscroll effect

[12people]

GTK has a nice glow overscroll effect (the "edge-overshot" effect).

To feel more native, it'd be nice if yaru supported this overscroll effect too.

Perhaps Flutter's [GlowingOverscrollIndicator](https://api.flutter.dev/flutter/widgets/GlowingOverscrollIndicator-class.html) — the old default for Material apps which would draw an ink splash on overscroll — might be a good starting point for such an implementation?

[12people]

Related to this issue: flutter/flutter#173513

[12people]

As an aside, I have a working version (though only with touch, as per the Flutter bug mentioned above) of a GTK-like overscroll effect. However:

• it's based on existing Flutter code (not sure what the copyright policy is on that)
• it's a very rough simulation of the GTK overshoot effect, not an exact match

Attaching it below:

gtk-overshoot.dart.zip

(Licensed as AGPL for now, but I have no problem relicensing it if someone here wanted to continue off of that work.)