getSite().getWorkbenchWindow().getSelectionService().addSelectionListener(...)

public class AdditionalWidgetInfoView extends ViewPart {

  public AdditionalWidgetInfoView() {
  }

  @Override
  public void createPartControl(Composite parent) {
    getSite().getWorkbenchWindow().getSelectionService().addSelectionListener("org.eclipse.wb.core.guiEditor", new ISelectionListener() {
      @Override
      public void selectionChanged(IWorkbenchPart part, ISelection selection)
      {
        TextSelection textSelection = (TextSelection)selection;
        System.out.println("text: " + textSelection.getText());
        
        IWorkbenchPart designerEditor = (DesignerEditor)part;
        designerEditor.getSite().getSelectionProvider().getSelection();
      }
    });
    
    getSite().getWorkbenchWindow().getSelectionService().addSelectionListener(new ISelectionListener() {
      @Override
      public void selectionChanged(IWorkbenchPart part, ISelection selection)
      {
        // Evaluate the selection in the active editor.
          IWorkbenchWindow workbenchWindow = getSite().getWorkbenchWindow();
          if (workbenchWindow == null)
          {
            return;
          }
          IWorkbenchPage activePage = workbenchWindow.getActivePage();
          if (activePage == null)
          {
            return;
          }
          IEditorPart activeEditor = activePage.getActiveEditor();
          if (activeEditor instanceof DesignerEditor)
          {
            DesignerEditor designerEditor = (DesignerEditor) activeEditor;
            @SuppressWarnings("restriction")
          TextSelection textSelection = (TextSelection)designerEditor.getSelectionProvider().getSelection();
            int startLine = textSelection.getStartLine();
          System.out.println("Element selected, line: " + startLine);
          }
      }
    });
  }

  @Override
  public void setFocus() {}
}