Add a filled/unfilled effect to the icons using svg properties

[fxedel]

Like in https://github.com/googlecodelabs/polymer-first-elements/blob/master/icon-toggle-finished/icon-toggle.html, add a possibility to toggle between an active fill or stroke property.

The element's property iconActive wouldn't be needed for switching between star-border and star or similar anymore. But it's still useful for cloud-off and cloud-queue, for example.