Investigate using Gradle as a build tool

[fwcd]

It would be nice if we could use Gradle here too, both for consistency with the other projects (KLS, KDA) and to simplify the build process, since e.g. the copying of the JARs in build.sh could be handled by Gradle, which is quite flexible in this regard.

I am not sure, however, how well building Eclipse plugins with Gradle is supported though.

Some useful resources:

• https://synesty.com/de/explore/blog/migrating-from-eclipse-tycho-to-bndtools-osgi/
• https://github.com/akhikhl/wuff