Play application uses Java/Scala cross-compilation

This is more a question than a feature request. Currently, the Play application only configures the Scala compiler. It does not point the Java compiler to the Play sources so it will always say `NO-SOURCE`. The Scala compiler takes on compiling both source types, Scala and Java source files.

In turn users will have to add additional Java source directories to the Scala source set. In noticed that in one of the samples.

```
sourceSets {
main {
scala {
srcDir 'additional/java'
}
}
}
```

If we wanted the Java compiler to take on some work then we'd probably have to configure the compilers for cross-compilation.