Bug 40788 [plan item] Configurable Next/Previous actions
Toolbar drop down, or editor specific control. Needs to be coordinated with Platform UI.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/JavaEditor.java
if (provider == null) {
			ReferenceProviderDescriptor defaultDescriptor= editorPlugin.getDefaultProvider();
			if (defaultDescriptor != null) {
				provider= defaultDescriptor.createProvider();
				if (provider != null) {
					provider.setActiveEditor(this);
					if (!provider.isEnabled()) {
						provider.dispose();
						provider= null;
					}
				}
			}
		}
		return provider;
	}
	/**
	 * Returns the annotation model associated with the document displayed in the 
	 * viewer if it is an <code>IAnnotationModelExtension</code>, or <code>null</code>.
	 * 
	 * @return the displayed document's annotation model if it is an <code>IAnnotationModelExtension</code>, or <code>null</code> 
	 * @since R3.0
	 */
	private IAnnotationModelExtension getModel() {
		ISourceViewer viewer= getSourceViewer();
		if (viewer == null)
			return null;
		IAnnotationModel m= viewer.getAnnotationModel();
		if (m instanceof IAnnotationModelExtension)
			return (IAnnotationModelExtension) m;
		else