title: JTable with cell selection and multiple interval selection shows incorrect selection
body: <p>I'd like to have a table where people can freely select cells.</p>
  
  <p>For this I've used the ListSelectionModel.MULTIPLE_INTERVAL_SELECTION and <em>table.setCellSelectionEnabled(true)</em>.</p>
  
  <p>I have 2 problems:</p>
  
  <ul>
  <li>The method <em>table.addRowSelectionInterval</em> doesn't select any cells</li>
  <li>If you select 2 blocks, the cells having the same rows and columns selected are also selected. e.g. select B2-D4 and then F7. This will also select B7-D7 and F2-F4.</li>
  </ul>
  
  <p>Here is the program to reproduce these problems:</p>
  
  <pre><code>import java.awt.BorderLayout;
  import javax.swing.*;
  
  public class JTableCellSelection {
      public static void showDemo(JComponent demo, String title) {
          JFrame mainFrame = new JFrame();
          mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          mainFrame.setTitle(title);
          JPanel contentPanel = new JPanel(new BorderLayout());
  
          contentPanel.add(demo);
  
          mainFrame.add(contentPanel);
          mainFrame.pack();
          mainFrame.setVisible(true);
      }
  
      public static void main(String[] args) {
          JTable table = new JTable(10, 10);
          table.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION);
          table.setCellSelectionEnabled(true);
          table.addRowSelectionInterval(6, 7); // Select 2 lines
          showDemo(new JScrollPane(table), "Select a block and some rows");
      }
  }
  </code></pre>
  
  <p>It feels like I will have to check the selection by myself and not rely on the isSelected of the table cell renderer.</p>