[output] Add support for click-events in i3

The i3 output now has a separate thread that continuously monitors stdin
for incoming click events. The generic output class also has methods for
registering callbacks (as commands). For now, by default, scroll events
will be used to emulate the next/previous workspace.

bumblebee/modules/disk.py

@@ -25,6 +25,9 @@ class Module(bumblebee.module.Module):
 
         return "{} {}/{} ({:05.02f}%)".format(self._path, bumblebee.util.bytefmt(self._free), bumblebee.util.bytefmt(self._size), self._perc)
 
+    def instance(self):
+        return self._path
+
     def warning(self):
         return self._perc < 20