Ugly fix to handle mpv:// url procotol

If url is something like mpv://http://some.url when clicking the URL
the : in http disappear, so in mpv the URL is not valid and the video
does not play. As this is just a quick test I removed http:// in url,
so append it in the code.

Shame on me.

2 videos to try :
http://repo.whine.fr/q3/__MPV-URL-HANDLER-TEST.html