mpv.desktop: add KDE file handlers

Currently KDE will copy a media file into a temporary folder instead of
trying to stream it if a KIO slave location file is started. This change
will tell KDE to make mpv try to directly play the file. Perhaps the
proper flags should be added according to the individual enabled
features of the build but I suggest that be for the future.

Signed-off-by: wm4 <wm4@nowhere>