Fix issue: using a CSS style directive in a widget constructor parameter erases any style already set on the widget node in buildRendering.

If a widget W is setting some CSS styles in its buildRendering method (for example, this.style.display = "block";), those styles settings are lost if redefining style in the argument of the constructor, as in:

```
var w = new W({style: "color: red;"});
```

because delite/register is replacing the style.cssText property of the widget node with the string declared in the constructor parameter.

I think that delite/register should add the new styles directive to the existing one instead of replacing it.