Pass the "output" object to the modules' constructor to allow them to
define their own callbacks.
Any user-provided callbacks take precedence and override those of the
module.
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.
Add a new module "disk" that takes an optional parameter (the path) and
displays free & total disk space, along with the usage percentage.
Also, added Tunnel/VPN support to the themeing of the "net" module.
