support mobile mode scrollbar
Summary:
when on mobile (or just "tabletmode" the scrollbar goes to "not interactive"
in that case it draws like a typical mobile scroll indicator, overlayed to the
view and disappearing
tough, it still shows a couple of pixels line when not in use so the users always
knows where in the view they are
Test Plan:
desktop:
mobile:
Reviewers: Plasma, VDG, Kirigami, ngraham
Reviewed By: VDG, ngraham
Subscribers: broulik, plasma-devel
Tags: Plasma
Maniphest Tasks: T9126
Differential Revision: https://phabricator.kde.org/D26654