Remove file encoding guessing from DATFile

[pixeldesu]

With using Stream in the constructor we can't even pass a file name anymore, aside that we've been guessing it anyway.

Move guessing of encoding to PangLib.Utilities instead, so people don't have to search for the encodings themself.

To Do:

• Add Encoding to DATFile constructor
• Move GetEncodingFromFileName to a static helper inside PangLib.Utilities