Code formatter/reporter

We should have something to report on things like unused imports, trailing spaces etc.

I'm only familiar with the following ones:

* Checkstyle from https://github.com/wildfly/wildfly. I think here the rules are a bit "old" for what we want to do here. Those are my impressions from another project I worked on, but I can't remember why.
* Spotless, from another private project I worked on. I found this way too intrusive, forcing lots of code changes for trivial stuff. Then again I am not very familiar with it.

If anyone has some ideas that would be great.