[core/input] clear previous input registrations

make sure that for a given event (widget/object/module, whatever), only
a *single* input event per button can be registered at one time.

the problem otherwise is with modules that re-register their widgets
with the same IDs (cmus, spotify, etc.): Each time the widget is
re-created (each intervall, typically), it re-registers an input event,
creating an always longer list of callbacks being executed when the
button is clicked (not speaking of the memory leak this introduces).

fixes #668

bumblebee_status/core/event.py

@@ -8,6 +8,9 @@ def register(event, callback, *args, **kwargs):
 
     __callbacks.setdefault(event, []).append(cb)
 
+def unregister(event):
+    if event in __callbacks:
+        del __callbacks[event]
 
 def clear():
     __callbacks.clear()