Followup to commit dba3b12e - further preparation for loading files from...

Followup to commit dba3b12e - further preparation for loading files from https://www.eclipse.org/papyrus

umbrello/umlmodel/attribute.cpp
- In function load1 case m_SecondaryId.isEmpty() while-loop, if href is
  non empty then require href to contain "PrimitiveTypes" in addition to
  "#".  Reason: Papyrus concidentally uses the datatype name as the
  xmi:id, but only in the predefined UMLPrimitiveTypes.library.uml and
  JavaPrimitiveTypes.library.uml.
  Proper resolution of href would require nested loading of model files,
  which is not yet implemented for Papyrus.

umbrello/dialogs/codetexthighlighter.cpp
- In function keywords() return empty QStringList immediately if `pl' is
  `Reserved' (else we crash due to null pointer dereference).

umbrello/codegenerators/codegenfactory.cpp
- In function createObject return null immediately if `pl' is Reserved
  (else we crash due to null pointer dereference).

CCBUG: 56184