Provide async version of tabris.app.registerFont()

Right now it's possible to register a font inside a web worker
```Javascript
global.onmessage = ()=>{
tabris.app.registerFont("helvetica", "helvetica.ttf");
};
```
but if you set the text property inside a TextView widget on the main thread the font will not be apply.

### Environment

- Tabris.js version: 3.3
- Device: Huawei P10 Lite
- OS: Android 8