[module/output] Re-enable user configurable mouse click events

This is now much nicer implemented to address issue #3. A user can now
have a configuration parameter mapped to a module instance (via the
module name or the instance name) with the value "left-click",
"right-click", etc., like this:

-m disk:home -p home.left-click="nautilus {instance}"

bumblebee/output.py

@@ -63,6 +63,8 @@ class Output(object):
         if module:
             module = module.replace("bumblebee.modules.", "")
 
+        if self._callbacks.get((button, module)): return
+
         self._callbacks[(
             button,
             module,