Visual plugin: Huffman-Encoding

[simlei]

Mainly: Fix UTF-8 problems

• revisit https://github.com/jcryptool/crypto/issues/287#issuecomment-610998143 to be sure
• see first stab at it in https://github.com/simlei/crypto/tree/huffmanUTF8

[simlei]

fixed (we decided to not convert the whole plugin to UTF-8-friendly code)