Improve scrollbars on touch

Hide the scrollbar background which makes it look like a desktop
scrollbar. Scrollbars are now by default non-interactive on touch.

[ChangeLog][QtQuickControls][ScrollView] Scrollbars are now
non-interactive on mobile/touch devices.

Task-number: QTBUG-37387
Change-Id: I63b6db9309a6e33140de0d26c8d4fd3e1bcfb832
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>