Don't skip content drawing for QtQuickControls SpinBox

If XPThemeData::noContent is set to true, the SpinBox from
QtQuickControls renders with transparent background, which is being
workarounded by simply adding white rectangle inside the QQC SpinBox,
which in turn breaks other themes.

This works in normal widget's QStyle because the spinbox there
is drawn with a frameless QLineEdit, which fills the background.

Setting XPThemeData::noContent to false for QtQuick items only makes the
QQC SpinBox render correctly with Windows Vista style.

Change-Id: I3229a2582dd1cad95c77e0961aaef502a8c6159a
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>