PC3 ScrollView: Avoid importing QtQuick.Controls.2

Normally, in a QQC2 style plugin's QML code you'd avoid importing QQC2
module, since that would cause an import loop. Same thing conceptually
applies here, except it wouldn't hang the app like a native QQC2 style
would -- it just looks wrong, and that's about it.

Instead, we can use T (QtQuick.Templates) for attached properties access,
and PlasmaComponents3.* for actual implemented components.