Conform to XDG Base Directory Specification

[jplitza]

It would be nice if Nagstamon would conform to the XDG Base Directory Specification. Basically, this would be accomplished by not putting its configuration in $HOME/.nagstamon, but instead in $XDG_CONFIG_HOME/nagstamon, falling back to $HOME/.config/nagstamon if $XDG_CONFIG_HOME is unset, and putting its pidfile in $XDG_RUNTIME_DIR/ (fallback is unclear here, maybe /tmp?). The latter would also have the benefit that on a system with shared NFS homes, launching multiple instances on different machines would be possible.

(NB: To strictly conform to the specification, Nagstamon would have to search for its configuration in a couple more directories)

[mprenditore]

Indeed, nice advice... I'm fixing something else at the moment but I'll look at it soon. :)

[HenriWahl]

Interesting... but only for Linux systems I guess, Windows and MacOS should not use this, even if there might be other defaults making more sense than $HOME/.nagstamon. As far as I remember there are even some configuration related classes in Qt5. Would be cool otherwise for a one-day-far-away coming CLI version not to have it too desktop-centric.