sound_play: Added functions to play files relative to a package path

I have added a new command PLAY_FILE_FROM_PKG that allows to play wave files relative to a package path (e.g. playWaveFromPkg("sound_play","sounds/BACKINGUP.ogg") will play the sound $SOME_PATH/sound_play/sounds/BACKINGUP.ogg). So no knowledge about the absolute path to the file is necessary. The relative path is given as arg and the package name as arg2.