Have LMMS follow the XDG Base Directory Spec

Enhancement Summary

This would have LMMS store all its files in accordance with the XDG Base Directory Specification, namely the configuration file that is currently stored at ~/.lmmsrc.xml

Under the XDG spec, a suitable location for storing this file would be $XDG_CONFIG_HOME/appdir which on most systems would point to ~/.config/appdir

Justification

This would not only bring LMMS in line with the standard for linux applications but would also help the user by decluttering their home folder and provide a common, standards based means of locating the file under linux systems.

Further details on the base directory spec can be found here: specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html and here: wiki.archlinux.org/title/XDG_Base_Directory

Mockup