ScrollablePage: Workaround Qt flickable inertia being really low

See teams/plasma-mobile/issues#273

The default flick deceleration in Qt 6 was changed from 1500 -> 5000 in Flickables making scrolling quite painful since there is little inertia.

They introduced a platform hint to customize it, however from my understanding we can't set that on the X11 and Wayland Qt platforms (I attempted to with plasma/plasma-integration!133)

This MR introduces a workaround for now so that scrolling isn't painful on a touchscreen for pages.

I reported the platform customization issue here: https://bugreports.qt.io/browse/QTBUG-121500