[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/input.py

@@ -50,6 +50,7 @@ def __execute(event, cmd, wait=False):
 def register(obj, button=None, cmd=None, wait=False):
     event_id = __event_id(obj.id if obj is not None else "", button)
     logging.debug("registering callback {}".format(event_id))
+    core.event.unregister(event_id) # make sure there's always only one input event
     if callable(cmd):
         core.event.register(event_id, cmd)
     else: