Add a way to keep the PID file and socket out of the config directory

[Hubro]

I want to symlink my sc-controller config directory to my Dropbox, so I can have a running backup and reuse my config seamlessly on multiple PCs.

However, the PID and socket files are not configuration, so I don't want them to be backed up. They will only cause conflicts between instances of Dropbox on different computers.

Instead, these files should probably be created in $XDG_RUNTIME_DIR:

➜ echo $XDG_RUNTIME_DIR
/run/user/1000

Even if you would like to keep the defaults the way they are, having a way to configure the location of these files would be very helpful.

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Off topic, I want to add that this is one of the most amazing open source projects I have ever seen. My jaw dropped when I first launched SC Controller and saw all the configuration options, plus the fact that it worked perfectly right away out of the box.