Allow opacity key in stylesheet to update fill and stroke opacities

[ba01ei]

In the stylesheet of an SVG, we can have opacity, fill-opacity, and stroke-opacity. When opacity is used, it should affect both fill and stroke if fill-opacity and stroke-opacity are absent. Previously SVGView only supported fill-opacity and stroke-opacity from the stylesheet but not opacity.

Also added a sample SVG file to demonstrate that, and included it in the unit test.