[core/input] Use util.cli to execute commands

If the registered callback is not callable (i.e. no Python method),
assume it's a command that should be invoked via a shell.

core/input.py

@@ -1,4 +1,5 @@
 import uuid
+import util.cli
 
 LEFT_MOUSE = 1
 MIDDLE_MOUSE = 2
@@ -10,7 +11,8 @@ callbacks = {}
 
 class Object(object):
     def __init__(self):
-        self._id = uuid.uuid4()
+        super(Object, self).__init__()
+        self._id = str(uuid.uuid4())
 
     def id(self):
         return self._id
@@ -31,5 +33,7 @@ def _invoke(event, callback):
     for cb in callback.get(event['button']):
         if callable(cb):
             cb(event)
+        else:
+            util.cli.execute(cb, wait=False)
 
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