[core] Fix callback registration ("shadowed" events)

Until now, as soon as a widget registered *any* callback, the default
callbacks (e.g. scroll up/down to go to next/previous workspace) didn't
work anymore, as there was a better match for the general registration
(even though not for the button).

To fix this, merge the callback registration into a flat registration,
where a key is calculated from the ID of the registrar and the
registered button.

see #23

bumblebee/modules/battery.py

@@ -22,6 +22,7 @@ class Module(bumblebee.engine.Module):
         battery = self.parameter("device", "BAT0")
         self._path = "/sys/class/power_supply/{}".format(battery)
         self._capacity = 100
+        self._ac = False
 
     def capacity(self):
         if self._ac: