Simplify Makefile, remove Xephyr target.

It is *one* source file and *one* header file. No need to get
megalomaniac with the Makefile either.

Xephyr is removed for the following reasons:
* The WM is slowly getting a bit more complex, Xephyr starts bugging.
* xinitrc will run dispwin to calibrate the screen (if $USER=ayekat),
  which doesn't work with Xephyr (see next commit).
* We want to get a feeling for how it works on a *real* X display.