JBIDE-15839 - Exception while saving Provider in JAX-RS application

Fixed the problem which occurred only when the problem severity as set to
'ignore' level. In that particular case, no IMarker would be created, thus
returning a null reference, which lead to NPE.

Added JUnit tests to cover all ways to create JAX-RS Markers.
JBIDE-15908 - Problem decorator is not removed after error was fixed

Bug was that the problemLevel on each JAX-RS Resource Method would not
be reset at the beginning of the validation, and during validation, the
highest problem level is kept. Thus, during the next validation (fixing
the problem), the problem level would remain at its highest (previous)
value.

Added tests to verify the correct behaviour on all validators.