Update Dimens class

Replace 

```
class Dimens {
  static const List<double> buttonTopDimens = <double>[28, 0, 28, 8];
  static const List<double> buttonBottomDimens = <double>[28, 8, 28, 0];
  static const List<double> buttonDimens = <double>[10, 0, 10, 0];
  static const List<double> none = <double>[0, 0, 0, 0];
}
```

with enumeration (check Wallet code)
