Don't re-add existing styles to SVGShapeElement

When calling `SVGShapeElement.reloadShapes()`, `searchShapes()` will
correctly reconciliate modified and added shapes and reuse their
elements. However, it will call `setElementStyles()`, which will add all
styles unconditionally to `this.stylesList`, even if the styles were
already in there.

Modify `setElementStyles()` to check for the existence of a particular
style before adding it.