Make jbpm-work-items repository compile with JDK 11

Currently the jbpm-work-items repository isn't compilable with JDK 11. There is a bug with annotation processor when used with JDK 11 which compiles to JDK 8 source and target. It canot properly parse an annotation which has an array as one of its parameters. If an import of that annotation is provided in the annotated file, suddenly everything will start working. Tests will be runnable with JDK 11 as well, just 2 modules won't be:

• mavenembedder-workitem because there are some issues with Eclipse JDK compiler
• karaf-itests because these tests use pax-exam-junit4 dependency which seems to be not working with JDK 11 in the version we use now