Gradient fix

Added rendering of gradientTransform, fixed gradient position ( not working since big commit of svg import ) fixed use of xlink:href in gradients, stop opacity 0% was omitted. Fixed toSVG for gradients.
Added gradients for stroke property.

Related to issue #1597
We need to use format like this:

```
/**
 * ...
 * @type Number
 * @default
 */
```