Mpris integration per Jas

Adds mpris integration for certain sites using a Firefox Webextension.
It allows you to have proper media key support for webplayers like youtube and spotify.
Only linux uses mpris so this extension will only ever work on linux!

The extension currently works with the following sites

• Spotify (open.spotify.com)
• Youtube (youtube.com)

Prerequisites
The following software is required to use this project.

• A fairly recent version of firefox (anything above version 64.0 should be fine)
• glib, dbus (install using your package manager)
• Something to interact with mpris players (like playerctl or gnome shell)

Installing

1. Install the webextension
2. Install the native host
   • Compiled version from here.
   • Or compile it yourself (see below)
3. Add a native manifest file
   • Copy the mpris_integration_native.json file to ~/.mozilla/native-messaging-hosts/ and replace the path key with the location of the native host binary.

Please see the github README for more info and how to compile this extension and the native host yourself.