Issue 495: Configure plugins menu should be disabled if there are no plugins to be configure.

git-svn-id: http://svn.dmdirc.com/trunk@2489 00569f92-eb28-0410-84fd-f71c24880f

src/com/dmdirc/ui/swing/MainFrame.java

                 pluginList.put(mi, plugin.getClass().getName());
             }
         }
+        
+        if (menu.getItemCount() == 0) {
+            menu.setEnabled(false);
+        }
     }
 
     /**