Add File.java to forbidden APIs

This commit cuts over all of core (not quite all tests) to java.nio.Path
It also adds the file class to the core forbidden APIs to prevent its usage
Not related to this change but we could write this:

```java
log("Installed plugins in " + environment.pluginsFile().toAbsolutePath() + ":");
if (plugins == null || plugins.length == 0) {
    log("    - No plugin detected");
} else {
```

It could help people to have a better understanding on plugins location. (IIRC I saw this request on the mailing list).