[modules] Add default click events to various modules

* cpu+memory: Open "gnome-system-monitor"
* disk: Open nautilus
* pulseaudio: Mute/unmute, open "pavucontrol" on right-click, raise/lower
  volume on mouse wheel up/down

bumblebee/modules/cpu.py

@@ -15,6 +15,9 @@ class Module(bumblebee.module.Module):
         super(Module, self).__init__(args)
         self._perc = psutil.cpu_percent(percpu=False)
 
+        output.add_callback(module=self.__module__, button=1,
+            cmd="gnome-system-monitor")
+
     def data(self):
         self._perc = psutil.cpu_percent(percpu=False)