JBIDE-17296 - Missing warning decorator on the "JAX-RS Web Services" node when no application is defined

Also includes fixes for:
- JBIDE-17078 - JAX-RS Problems doesn't appears after the JAX-RS Support is activated
- JBIDE-17771 - Trigger validation when JAX-RS nature is added on a project
- JBIDE-18068 - JAX-RS Explorer doesn't have "expand" icon
- JBIDE-18114 - Invalid state of JAX-RS Web Services node in Project Explorer after JAX-RS Support removal