MOC should use the XDG directories

Hello!

The use of the $HOME/.moc directory with everything stored in it is now deprecated by the XDG specification.

Here are the needed changes to support that spec:

• the file config should be moved to $XDG_CONFIG_HOME if set, else $HOME/.config;
• the current cache dir should be renamed $XDG_CACHE_HOME if set, else $HOME/.cache;
• pid and socket2 should be in $XDG_RUNTIME_DIR if set, else in something like /tmp/moc-$UID;
• all other files should be moved to $XDG_DATA_HOME (or $XDG_CONFIG_HOME? I’m not sure);
• $XDG_MUSIC_DIR, if set, should be the directory displayed by default, else $HOME, as now.

Thanks,
Link Mauve