JBIDE-15825 - JAX-RS problems don't show up after target runtime is set

Main changes:
    - handle changes at the project level when the classpath changed: rebuild the whoe JAX-RS metamodel to be process all the changes
    - remove all JAX-RS markers of previous JAX-RS elements when rebuilding the while JAX-RS metamodel in a separate Job while the resource tree
      is locked.

    Related changes:
    - added some unit tests
    - refactored a unit test class
you're right, I created it but did not use it. Will remove it in an updated version of the PR