Add mpv.desktop, let Makefile install the desktop file and the icons

The mpv.desktop file is taken from the Arch package [1]. It appears to
be based on the mplayer2 git mplayer.desktop file (e.g. very similar
MimeType field), with minor modifications applied by Arch package
maintainers.

Note that for now, this doesn't show a terminal (Terminal=false), which
might not always be ideal. For example, if the file is audio only, or
if VO initialization fails for some reason, mpv will run in the
background and play audio without showing a window. But users prefer
running it without terminal, and don't want to play audio files with
it.

Maybe a --force-window option will be added in the future, which would
always create a VO window, and compensate for these issues.

[1] https://aur.archlinux.org/packages/mpv-git/
manpage: remove bogus space in example usage