Add previous song option to editor music screen

This is just a small quality-of-life feature - it's annoying to have to
press ACTION 15 times in order to cycle back through to the previous
song.

desktop_version/src/Game.cpp

@@ -6417,6 +6417,7 @@ void Game::createmenu( enum Menu::MenuName t, bool samemenu/*= false*/ )
         break;
     case Menu::ed_music:
         option("next song");
+        option("previous song");
         option("back");
         menuyoff = 16;
         maxspacing = 15;