desktop-shell: add option to avoid creating the panel

This option is so we can disable showing any panel at all. The default
is to continue showing the panel and no example is added to weston.ini
because it's an uncommon request.

Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/man/weston.ini.man b/man/weston.ini.man
index 3d8eef9..22f9f0d 100644
--- a/man/weston.ini.man
+++ b/man/weston.ini.man
@@ -186,6 +186,11 @@
 .fi
 .RE
 .TP 7
+.BI "panel-location=" top
+sets the location of the panel (string). Can be
+.B top,
+.B none.
+.TP 7
 .BI "locking=" true
 enables screen locking (boolean).
 .TP 7