Require an explicit --xephyr option

Require an explicit --xephyr option

Instead of starting Xephyr automatically, require --xephyr to be
passed explicitly.

This makes the operation easier to understand and has the benefit
of allowing running in Xephyr mode when some other window manager
(like gnome-shell!) is running. We also want to emphasize that
Xephyr is a development tool, and not a good preview of the
user-interface.

http://bugzilla.gnome.org/show_bug.cgi?id=592881