JBIDE-15039 - JAX-RS Problem decorator not shown on the Project Explorer node

Main refactoring/changes in this PR consist in:
- compare the problem level of each JAX-RS element before vs after the validation, and
in case of change, notify the metamodel. The metamodel will look for all affected Endpoints and
send 'problem level change' notification to the listeners (here, the UI). In turn, the UI will *update*
the associated node (update -> no refresh for sub nodes) to get a proper image decorator.
- same logix with the metamodel itself, except that the public problem level includes the problem level of the
JAX-RS elements, which means that the 'JAX-RS Web Services' node now reflects the whole Metamodel and thus, displays the
global problem level.
This new UI refresh strategy also reduces the number of times refresh occurred (there's now a single refresh per affected element, even
if this element has multiple problems).

- also fixed a problem where the UI listener would only be registered once the "JAX-RS Web Services" node was expended