lxqt-config-session/xdg-user-dirs: resetting of paths featuring characters unsupported by current locale

[pmattern]

lxqt-config-session is resetting paths according to the xdg-user-dirs specification to their default value /home/<user> if these comprise characters unsupported by the locale lxqt-config-session is using.

Steps to reproduce

• Make sure none of the directories in $HOME as defined by xdg-user-dirs exists and binaries xdg-user-dir[s-update] are available.
• Launch an LXQt session with LANG=de_DE.UTF-8. XDG_PUBLICSHARE_DIR is set to /home/<user>/Öffentlich, hence to a string comprising a German Umlaut. Everything is fine if lxqt-config-session is launched with the current locale de_DE.UTF-8, any other UTF-8 locale or not at all.
• Launch the LXQt session as a whole or lxqt-config-session only with LANG=C and select "Locations" in left pane of lxqt-config-session. Path "Public Share" at the right will be empty meaning it gets reset to its default value /home/<user> when lxqt-config-session is quit. (The inconsistency empty path vs. /home/user is being discussed in #780.)

It's understandable that lxqt-config-session can't cope with characters unsupported by the locale it's running in. But it should just leave the paths as is and not start tweaking them.

Binary xdg-user-dirs itself does not create any directories corresponding to a locale that's different from the one that was valid when ~/.config/user-dirs.dirs was initially written as long as all directories corresponding to that initial locale persist.