The public member variables for opacity, saturation and brightness
are removed in favor for getter and setters. The variables are
moved into a private class. Those are now qreal instead of double.

To make usage inside the effects easier a multiply method is added
which multiplies the current value with passed in factor and returns
the new value in a functional programming style.

This commit is the top-most of a patch series to refactor
ScreenPaintData and WindowPaintData. Other related commits are:
* 0811772
* ebdc7ec
* 2c8dd8d
* 7699726
* 68e0201
* 611cb09

REVIEW: 105141
BUG: 303314
FIXED-IN: 4.10