Improvements to development

[manuelprinz]

Hi, and thank for this project! It helped me quite a bit.

While working on a fix for the issue I has (#125) I ran into problems which turned out to be because I did not know that I needed to build the front-end first, as it had no dependency on the "jar" target. The missing index.html made the tester fail although everything should have worked. It would be nice to have the dependency explicit in the build file or at least give a hint in the documentation.

Also, the NPM targets do not run if they are run from the IDE when NPM is not in PATH. Maybe this is an IDE problem only.

Also, a couple of things happened in the Kotlin ecosystem. Maybe Kotlin/JS integration would be an option? I should be possible to manage NPM/Webpack from Gradle. (I am not too deep into that, unfortunately.)

If you can give me some pointers on what to include in the documentation I'd be happy to create a MR.