Error/warning if output folders contain non JNIgen files

[dcharkes]

If I remove a class from the input, and it's the last one in the package currently (and after https://github.com/dart-lang/jnigen/pull/98 just by itself), the old file will linger around.

I think we should delete all contents in the output folder before generating.

Moreover, we should probably check if there are any files that were not created by jnigen, and error if there are any.

• [ ] Make all generated files recognizable (maybe they already are)
• [ ] Error if there are non JNIgen files in output folders
• [ ] Empty the output folders before generating

[mahesh-hegde]

Make all generated files recognizable (maybe they already are)

Currently they are not. User provided preamble goes before our boilerplate // Auto generated, DO NOT EDIT..

It's a matter of splitting the boilerplate again.