lxqt-config-locale: derive default values from session locale

[pmattern]

If lxqt-config-locale gets launched using a locale different from the session's one, e. g. by running LANG=en_GB.UTF-8 lxqt-config-locale in a session that's run with LANG=de_DE.UTF-8, the former locale gets taken on as default value in the GUI.
Closing the UI doesn't throw any message and hence suggests the English locale gets set. But this doesn't apply. Rather, no locale settings are changed at all.

Launching lxqt-config-locale in a locale that's different from the session's one is probably an approach not taken too frequently on production systems, it's rather done to perform tests. Yet I think this is something that should get fixed.

[Vladimir-csp]

Confirming this

[Vladimir-csp]

Actually, this may be kind of a duplicate to #891. If lxqt-config-locale would look into session.conf for settings, then problem solved.