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