[core] Add input processing

Create infrastructure for input event handling and add i3bar event
processing. For each event, callbacks can be registered in the input
module.
Modules and widgets both identify themselves using a unique ID (the
module name for modules, a generated UUID for the widgets). This ID is
then used for registering the callbacks. This is possible since both
widgets and modules are statically allocated & do not change their IDs.

Callback actions can be either callable Python objects (in which case
the event is passed as parameter), or strings, in which case the string
is interpreted as a shell command.

see #23

bumblebee/modules/cpu.py

@@ -11,6 +11,8 @@ class Module(bumblebee.engine.Module):
             bumblebee.output.Widget(full_text=self.utilization)
         )
         self._utilization = psutil.cpu_percent(percpu=False)
+        engine.input.register_callback(self, button=bumblebee.input.LEFT_MOUSE,
+            cmd="gnome-system-monitor")
 
     def utilization(self):
         return "{:05.02f}%".format(self._utilization)