Installer should respect XDG_USER_DIRS on Linux

[modelmat]

On Linux the installer installs to ~/Desktop always. https://github.com/wpilibsuite/WPILibInstaller-Avalonia/blob/master/WPILibInstaller-Avalonia/ViewModels/InstallPageViewModel.cs#L742

If xdg-user-dirs is used to place it somewhere else, the installer creates the ~/Desktop directory and puts it in there. Ideally it should use xdg-user-dir DESKTOP to find the location of the desktop, then default to ~/Desktop. (note that the environment variables are not always set and may need to be fetched from ~/.config/user-dirs.dirs)

[PeterJohnson]

It's been pointed out that xdg-user-dir is not installed by default, so it will be necessary to implement its logic, potentially by embedding (part of) this as a /bin/sh command line.

#!/bin/sh

test -f ${XDG_CONFIG_HOME:-~/.config}/user-dirs.dirs && . ${XDG_CONFIG_HOME:-~/.config}/user-dirs.dirs
if [ "x$1" = "xDESKTOP" ]; then
  eval echo \${XDG_${1}_DIR:-$HOME/Desktop}
else
  eval echo \${XDG_${1}_DIR:-$HOME}
fi