Use application directory for storing files used by mpv #1995

[krysanify]

@Secozzi

Organizes the files inside a mpv subdirectory, as well as creating separate directories for fonts, scripts, script-opts, etc. Hopefully this fixes the issue of ffmpeg being unable to read the cacert.pem file for some people. Also disables ytdl

Since mpv will now always read from the application directory, I'm assuming there's no need to ask for external storage permission when copying over user files