QScrollBar: unpressed scroll buttons look pressed on QWindowsStyle

QWindowsStyle considers QStyle::State_On as pressed, whereas QMacStyle uses the same state for transient scrollbars. Perhaps it would be best to avoid this confusions by having a dedicated state for transient scrollbars, but it's a bit late for that in Qt 5.0.0. Thus, to indicate transient scrollbars, QScrollBar::initStyleOption() should set QStyle::State_On only when the current style actually supports transient scrollbars.