minimalist open in mpv wot binarynoise, Joshix

A minimalist extension to open the current website or a link in mpv.
You only need to put a .desktop file to `~/.local/share/applications/mpv-scheme-handler.desktop`.

And if you don't have that file installed, you get a shell-script to put it into place.
No more "nothing happens" and no-one knows why!

Inspired by Open In mpv by Leonardo and ff2mpv by yossarian, which require additional binaries installed but don't tell you when they are missing.