[output] Using "instance" for callback registration doesn't make sense

"instance" is actually better suited to be an argument to the callback
(which it already is) than part of the key for callback lookup.

bumblebee/output.py

@@ -4,30 +4,21 @@ class Output(object):
         self._theme = theme
         self._callbacks = {}
 
-    def add_callback(self, cmd, button, module=None, instance=None):
+    def add_callback(self, cmd, button, module=None):
         self._callbacks[(
             button,
             module,
-            instance
         )] = cmd
 
     def callback(self, event):
         cb = self._callbacks.get((
             event.get("button", -1),
             event.get("name", None),
-            event.get("instance", None)
-        ), None)
-        if cb is not None: return cb
-        cb = self._callbacks.get((
-            event.get("button", -1),
-            event.get("name", None),
-            None
         ), None)
         if cb is not None: return cb
         cb = self._callbacks.get((
             event.get("button", -1),
             None,
-            None
         ), None)
         return cb