Respect the XDG_CONFIG_HOME standard for .livebookdesktop.sh

[sdball]

Currently livebookdesktop.sh is placed directly in $HOME which is not ideal. It'd be nicer to use $XDG_CONFIG_HOME, or $HOME/.config/livebook/..., or failing those $HOME/... could be a last resort. If placed in .config/livebook then the standard is to not make dotfiles in the .config directory e.g. $HOME/.config/livebook/desktop.sh

[josevalim]

Because we expect the user to create those, we decided to go with $HOME, because if it changes per OS or per config, as the XDG_CONFIG_HOME allows, it makes it very hard for us to write documentation or references about it.

[sdball]

Could its location at least be controlled by an ENV variable for those of us that strictly manage $HOME contents?

[josevalim]

Yes, please do send a PR. We would need to support it on both Windows and MacOS. :)