Bug#835672: xfce4-power-manager: If installed without xfce4-session, fails to lock screen due to missing xflock4 script

[Akbarkhon Variskhanov <akbarkhon.variskhanov@gmail.com>]

Control: tags -1 upstream moreinfo
Control: wishlist -1 minor

That's how it's intended to be. See
https://bugzilla.xfce.org/show_bug.cgi?id=12603

If you check your ~/.xsession-errors there should be a warning about
setting a LockCommand. The simplest fix is to actually set it via
`xfconf`:

$ xfconf-query --channel xfce4-session --property /general/LockCommand
--set "lock_command_of_your_choice"

If it's not set, the following commands will be tried in the order specified:
xflock4
xdg-screensaver lock
xscreensaver-command -lock

If none succeed, the function xfce_screensaver_lock() returns FALSE,
which makes it look like it's useless.

Note that it's not supposed to work 100% outside Xfce4.