Support FontIcon widget

At least in winRT and web apps using font-icons instead of png/gif/jpg or even SVG icons get's increasingly common.

There are some very significant advantages:
- Infinite Scalability: No need to think about different image and screen resolutions.
- No additional resources: All you need is one font (which is either part of the OS or bundled with client).
- Customizable colors: Decide at runtime which color the icon should have.
- Consistent UI: Using one (or two) fonts per client gives your app have a look matching the OS.

Disadvantages:
- Monochromatic only

Windows and Android provide fonts for their platform, see [here](https://msdn.microsoft.com/en-us/library/windows/apps/jj841126.aspx), [here](https://github.com/winjs/winjs/tree/master/src/fonts) and [here](http://zavoloklom.github.io/material-design-iconic-font/icons.html#hardware). I couldn't find one for iOS/Mac/Apple, but there are free fonts we could use, like [this one](http://ionicons.com/), [this one](http://fortawesome.github.io/Font-Awesome/icons/) and [this one](https://icomoon.io/#preview-free). Those could also be used to fill in missing icons on the android/windows fonts.

We would have to find common icons the different fonts use and make them available under one keyword, e.g.

```
tabris.create("FontIcon", { glyph: "phone", ios_glyph: "#8657" } );
```

I think this feature would fit perfectly with our "easey-to-use" USP. Discuss!