disappearing scrollbars on mobile

Summary:
like other platforms, on mobile devices make scrollbars visible only when
dragging/flicking, behavior on desktop unchanged
it introduces a runtime dependency on kirigami, but was already kinda there
but broken as the Settings singleton was already used.
if this can't be done, i'll add something in plasmacore which reads as well the
QT_QUICK_CONTROLS_MOBILE environment variable

Test Plan: scrollbar disappearing with the variable set, normal behavior otherwise

Reviewers: Plasma, bshah, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D9287