Hidden settings for xfce4-session

[Jarno Suni]

Hello,

You can configure which buttons are visible in the logout dialog, but
many settings are not visible even in Settings Editor by default, let
alone documented.

There is an xml file settings/xfce4-session.xml where there are some
properties declared, but not in the shutdown sub-category. Should they
be added in the file? I have some properties shown in Xubuntu, but not
e.g. shutdown/ShowSwitchUser
(https://forum.xfce.org/viewtopic.php?pid=56285#p56285).

I added some properties in that file in my merge request concerning
xflock4, already. The proposed xflock4 script echoes some warnings in
standard error though, if it can not find the settings it would use
making it easy to adopt it even to older releases.