Fusion style: Use QStyleHelper::dpiScaled() where applicable.

When running unscaled on a high resolution display, some items
are too small. This patch converts most values returned by
QFusionStyle::pixelMetric() via QStyleHelper::dpiScaled(). Some
adjustments have been done to RadioButton, CheckBox, Slider and
SpinBox.

On systems with 96dpi, FusionStyle should still look exactly as
before this patch.

Task-number: QTBUG-40277
Task-number: QTBUG-38858
Change-Id: I2683a8a4db615d01c08f4d6f559417b995c0eb1a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>