Using syncplay with flatpak apps

[arvigeus]

1. Create a mpv.sh file with the following content:

#!/bin/bash
flatpak run io.mpv.Mpv "$@"

2. Make the file executable

3. Point syncplay to that file when asks you for a player

Should work with other supported players as well

[soredake]

@arvigeus can you add LD_LIBRARY_PATH= before flatpak command? It will fix #536