Respect XDG user documents directory

[SpotlightKid]

Please don't do that:

https://github.com/moddevices/mod-app/blob/063a88eaef4379bcaf30c1b2269988500cceb2dd/systray/main.cpp#L245

That will clutter the user's home directory with yet another directory, if the desktop language isn't english or the user changed the location of the DOCUMENTS folder.

Use xdg-user-dir DOCUMENTS everywhere instead.

[falkTX]

well yeah that is the point if you look just above that code line. the xdg stuff is still TODO.
the whole app initialization thing is very crude and ugly, just enough to get things started for now.

[falkTX]

Fixed in 503c5ed, now using Qt for this, by means of QStandardPaths::writableLocation(QStandardPaths::DocumentsLocation)