qml: do not consume events in scroll bar area if scroll bar does not show anything

This makes it possible to not consume the events when there is nothing shown. We can do this because in this scroll bar style when size is smaller than 1.0, the handle is always visible. So this change is mostly relevant when size is 1.0, which means viewport size covers the content size.

This is useful for situations such as, playlist height is equal or greater than the combined height of the items added to it. With this patch, clicking on the items where the scroll bar would be shown makes it possible for items to react to the events (such as, selecting or showing the context menu).

I did not use background.visible or contentItem.visible because they already depend on the control's visibility (as if parent is invisible the children would be invisible).

Request review @chub.