Suggestion: Use document icon

Previous installers used to use [mpv-document.ico](https://github.com/rossy/mpv-install/blob/master/mpv-document.ico) which looks much nicer on files than the stock mpv icon. Is there any reason this isn't the case here?

See:
https://github.com/mpv-player/random-stuff/tree/master/icon
https://github.com/mpv-player/mpv/commit/303096bcb2b5f52ea51d5d7ddb738689b08a0a2f
