Metadata as Class and Polymod fixes

[soushimiya]

What changes in this commit

• It handles metadata as classes(SongMetadata, ImageMetadata) similar to what Polymod does with ModMetadata. I created it to reduce the crap out of if(any != null){any = meta.any}.
• This change makes it easier to use placeholder values ​​and essentially use metadata anywhere. (For example, you can play a song using only the composer's name data.)

  Other fixes

• Fixed Util's readDirectory refusing to add files on some OS
• The method of enabling/disabling mods in Polymod has been replaced with the Polymod recommended method of using loadMods() and unloadAllMods(). (This is slightly faster as you don't need to call Polymod.init() again)