Add bizarrely named "mtime" to MPRIS.

Fixes issue #893

src/core/player.cpp

@@ -459,6 +459,7 @@ QVariantMap Player::GetMetadata(const PlaylistItem& item) const {
   AddMetadata("samplerate", song.samplerate(), &ret);
   AddMetadata("bpm", song.bpm(), &ret);
   AddMetadata("composer", song.composer(), &ret);
+  AddMetadata("mtime", song.length() * 1000, &ret);
 
   return ret;
 }

src/core/song.h

@@ -267,7 +267,7 @@ class Song {
     int skipcount_;
     int lastplayed_;
 
-    int length_;
+    int length_;  // Seconds.
     int bitrate_;
     int samplerate_;