libkeybinder2 and Kupfer settings patch

Title says it all. Though for further clarity:
* libkeybinder2 was packaged to make kupfer's setting keybinding work without having to assign the keybinding on a window manager level (which, if it was handled by the window manager, it'd be calling the kupfer command manually through that keybinding).
* The second commit introduces the python-keybinder2 dependency (since it's a highly recommended optional dependency) and includes a patch so clicking on the Settings menu item shows the settings.