Add type safety to internationalized messages

[acbeni]

Right now Labels are used to get hold of messages specified in the yaml files. That can be enhanced through generation of classes that contains all the keys of the yaml messages. the main gain from this is type safety of messages.
this will complement issue #684

[lauraluiz]

Example of accessing resources in Android: https://developer.android.com/guide/topics/resources/accessing-resources.html