Add scenes support

Add the possiblity to use scenes, e.g. in boards definition:

`{ 
	"boards" : [
    {
        "name" : "Soundboard",
		"folder" : "assets/",
		"scenes" : [
			{
				"name": "alert",
				"sounds": [
					...
				]
			}
		],
        "sounds" : [
            {`

If a scene is selected all sound clips of these scene are played automaticly.