Add support for DBus MPRIS and XDG integration

[tatokis]

This PR adds support for the MPRIS DBus spec, as well as desktop integration using XDG. Some examples:

$ dbus-send --print-reply --dest=org.mpris.MediaPlayer2.vgmplay /org/mpris/MediaPlayer2 org.freedesktop.DBus.Properties.Get string:'org.mpris.MediaPlayer2.Player' string:Metadata
method return time=1562058434.358467 sender=:1.17860 -> destination=:1.17862 serial=29 reply_serial=2
   variant       array [
         dict entry(
            string "mpris:trackid"
            variant                string "/org/mpris/MediaPlayer2/CurrentTrack"
         )
         dict entry(
            string "xesam:url"
            variant                string "file:///home/tatokis/%CE%95%CF%80%CE%B9%CF%86%CE%AC%CE%BD%CE%B5%CE%B9%CE%B1%20%CE%B5%CF%81%CE%B3%CE%B1%CF%83%CE%AF%CE%B1%CF%82/VGM/Mega%20Turrican/03%20-%20Stage%201-1.vgz"
         )
         dict entry(
            string "mpris:artUrl"
            variant                string "file:///home/tatokis/%CE%95%CF%80%CE%B9%CF%86%CE%AC%CE%BD%CE%B5%CE%B9%CE%B1%20%CE%B5%CF%81%CE%B3%CE%B1%CF%83%CE%AF%CE%B1%CF%82/VGM/Mega%20Turrican/Mega%20Turrican.png"
         )
         dict entry(
            string "xesam:lastused"
            variant                string "2018-01-04T12:21:32Z"
         )
         dict entry(
            string "xesam:genre"
            variant                array [
                  string "Video Game Music"
               ]
         )
         dict entry(
            string "xesam:album"
            variant                string "Mega Turrican"
         )
         dict entry(
            string "xesam:title"
            variant                string "Stage 1-1"
         )
         dict entry(
            string "mpris:length"
            variant                int64 188850333
         )
         dict entry(
            string "xesam:artist"
            variant                array [
                  string "Chris Hülsbeck"
               ]
         )
         dict entry(
            string "xesam:composer"
            variant                array [
                  string "Chris Hülsbeck"
               ]
         )
         dict entry(
            string "xesam:trackNumber"
            variant                int32 3
         )
         dict entry(
            string "xesam:discNumber"
            variant                int32 1
         )
         dict entry(
            string "xesam:useCount"
            variant                int32 0
         )
         dict entry(
            string "xesam:userRating"
            variant                double 0
         )
         dict entry(
            string "vgm:release"
            variant                string "1994/06/07"
         )
         dict entry(
            string "vgm:creator"
            variant                string "2ch-N"
         )
         dict entry(
            string "vgm:system"
            variant                string "Sega Mega Drive"
         )
         dict entry(
            string "vgm:version"
            variant                uint32 336
         )
         dict entry(
            string "vgm:loop"
            variant                int64 158760000
         )
         dict entry(
            string "vgm:chips"
            variant                array [
                  string "SEGA PSG"
                  string "YM2612"
               ]
         )
      ]