title: the focus sticks on the shell and it Traverses by tab key no more
body: <p>I made a simple shell contents two buttons with SWT.<br>
  after compiling the code below, when I press tab key on the shell several times, the focus traverses from the button one to the button two and next goes on the shell (plane); and despite pressing tab key, the focus stays there, it traverses no more.<br>
  while the shell's style is NO_FOCUS.<br>
  could someone please tell me how this issue should be solve?  </p>
  
  <p>I am working with: Windows 10 64-bit, Java 8 64-bit, swt v4.7.3, Eclipse neon (java SEE developers), </p>
  
  <pre><code>import org.eclipse.swt.SWT;
  import org.eclipse.swt.widgets.Button;
  import org.eclipse.swt.layout.RowLayout;
  import org.eclipse.swt.layout.RowData;
  import org.eclipse.swt.widgets.Display;
  import org.eclipse.swt.widgets.Shell;
  
  public class MainWindows {
  MainWindows() {
  Display display = new Display();
  Shell shell = new Shell(display, SWT.SHELL_TRIM | SWT.CENTER | SWT.NO_FOCUS | SWT.TOP);
  shell.setText("test win");
  shell.setLayout(new RowLayout());
  shell.setSize(500, 500);
  Button b1 = new Button(shell, SWT.PUSH);
  Button b2 = new Button(shell, SWT.PUSH);
  b1.setBounds(100, 100, 50, 50);
  b2.setBounds(200, 200, 50, 50);
  b1.setText("b1");
  b2.setText("b2");
  shell.open();
  
  while (!shell.isDisposed()) {
  if (!display.readAndDispatch()) display.sleep();
  }
  
  display.dispose();
  }
        static void main(String[] args) {
  MainWindows mainWindows = new MainWindows();
  }
  }
  </code></pre>