+ when configuring workspace preferences:
  + search for annotations with workspace scope
+ report not found annotation types
+ report duplicate annotation types of different kind (ambiguous)
+ cascade warning dialogs:
  - first report problems (not found / ambiguous)
  - only then check for incompatibilities
+ make detection more robust (single element, semi-qualified name)

Fixes https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/2303