Add protocol registration on Linux

[erri120]

On Linux systems that support freedesktop.org Interoperability specifications, it is very easy to add a URL scheme handler using either a Desktop Entry file or using the official utilities.

Example desktop entry:

[Desktop Entry]
Name=Steam (Runtime)
Comment=Application for managing and playing games on Steam
Exec=/usr/bin/steam-runtime %U
MimeType=x-scheme-handler/steam;x-scheme-handler/steamlink;

Example usage of the official utilities:

xdg-settings get default-url-scheme-handler mailto

xdg-settings set default-url-scheme-handler mailto thunderbird-beta-bin.desktop