This is a follow-up for the patch that removed weston-screensaver. The
aim is to clean up shell.c a little by removing non-essential
components. Vanilla Weston desktop is only a demo, external projects are
encouraged to create user-friendly desktop environments.

The support for launching a screensaver client and the protocol bindings
are removed. With them, all related configuration options are removed,
and the manuals are updated accordingly.

The screensaver protocol definition is left in desktop-shell.xml for
posterity.

This does not affect Weston's or desktop-shells ability to put screens
to sleep after inactivity. The inactivity timer continues to operate as
before. Also screen locking is unaffected.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Giulio Camuffo <giuliocamuffo@gmail.com>