Override implicit focus transfer when "F10" key is pressed

Java L&F has pre-defined hook that transfers the focus to the
menu of the RootPane.  Since the viewer doesn't have a menu
this just causes a loss of focus and subsequent unexpected
behavior of keyboard and mouse.

java/com/tigervnc/vncviewer/Viewport.java

@@ -102,6 +102,16 @@ class Viewport extends JPanel implements ActionListener {
         releaseDownKeys();
       }
     });
+    Action action = new AbstractAction() {
+      public void actionPerformed(ActionEvent e) { }
+    };
+    KeyStroke f10 = KeyStroke.getKeyStroke(KeyEvent.VK_F10, 0);
+    getInputMap(JComponent.WHEN_FOCUSED).put(f10, "requestFocus");
+    getActionMap().put("requestFocus", new AbstractAction() {
+      public void actionPerformed(ActionEvent e) {
+        requestFocusInWindow();
+      }
+    });
 
     setFocusTraversalKeysEnabled(false);
     setFocusable(true);