Add proguard config for Creatable

If a scope is only instantiated via the new Creatable method, proguard will strip the Impl with the current config. As a temporary workaround, all subclasses can be kept via: 
```
-keep class * implements motif.Creatable {
    <init>(...);
}
```

However long-term the solution is to use `-if` to pair the impl with the kept state of the interface.