PC3 ScrollView: Fix ScrollBar.active property

First, ScrollView, ScrollBar attached property and built-in Qt themes
are not really designed to handle nullable scrollbars. So we don't care
to support such cases either. There is a well-documented way to hide a
particular scrollbar: `policy: ScrollBar.AlwaysOff`.

Second, it was a careless typo or a mistake to bind scrollbars' active
property to themselves. Instead they should bind to each other, so that
they show up and disappear simultaneously on hover.