Add new package xdummy, a dummy X server.

This is actually a small script which just starts an X server without
any real display. Right now only needed for running the test suite of
the i3 window manager within the Nix chroot, but might be useful for
running other tests needing a DISPLAY.

Usage is just like a regular X server, so in order to start an instance
for display :666, you just run it like this:

xdummy :666

Signed-off-by: aszlig <aszlig@redmoonstudios.org>