[engine/input] Re-enable mouse button parameter bindings

Re-enable the possibility to define custom mouse actions by binding
commands to "<alias|module>.<left-click|right-click|...>". These
commands are then executed as shell commands.

fixes #30

bumblebee/input.py

@@ -9,6 +9,7 @@ import threading
 import bumblebee.util
 
 LEFT_MOUSE = 1
+MIDDLE_MOUSE = 2
 RIGHT_MOUSE = 3
 WHEEL_UP = 4
 WHEEL_DOWN = 5