Show the dialog just once when there are many unsaved sketches

Author: cmaglie

Diff for: app/src/cc/arduino/view/preferences/Preferences.java

@@ -254,6 +254,7 @@ public void mouseEntered(java.awt.event.MouseEvent evt) {
                                           tr("Can't enable external editor"),
                                           JOptionPane.INFORMATION_MESSAGE);
             externalEditorBox.setSelected(false);
+            return;
           }
         }
       }