Change SVG colors to not use RGBA (technically not allowed).

All our target browsers allowed us to use RGBA for a `linearGradient: stop-color`, but technically it's not in the SVG spec.  SVG defines `stop-opacity` and `stop-color` separately, and the `stop-opacity` is the only place where alpha values work.

When I tried to export a new copy of the Animation widget into Inkscape (which hasn't happened in a long time), the colors came out black because we're not following the SVG spec.

So, this pull request brings us back into compliance, but there should be no visible changes in the browser as a result.  In particular, the translucent parts of the Animation widget should be completely un-changed by this pull.