QmlDesigner.PropertyEditor: fixing step size for opacity

Change-Id: I461baba451c0aed3c34ec581bccce82f954df511
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner.PropertyEditor: moving the color logic into component

Change-Id: I3d90618f5826cf77cc9813fd3b31fbfdbd5b50e7
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>