Fix launching on Linux (proton)

[int-72h]

Finally figured it out - this approach seems to work. xdg-open steam -applaunch [appid] -game [game path] [args]

[fjuton]

Removed the redundant proton_map_to_depot variable since it wasn't used anymore. I'd probably unify command_line and url_launch too but it's a small nitpick. Other than that everything looks good to me (if the core concept of this commit works as intended ofc.)