gradle: build the projects with Gradle (v5)

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: add a (temporary) script to fully exercise the build

I keep having to look up all the targets...

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
findbugs: more effort and more reports

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>