add a background in ScrollView

Summary:
add a background in scrollview similar to what qwidget's scrollviews do
hide it by default since is not something that seems much in line with qqc2

the api to show it would be scrollview.background.visible = true

which is not really public/nice but it would work without adding extra api

Test Plan: tested with the new design of qml kcms

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8526