Add UTF-8 support to VMD

[Sakura-Luna]

MMDtools directly discards unsupported characters when exporting VMDs, but it's feasible to keep it and it doesn't affect MMD loading in tests. Due to the size of UTF-8, longer names can be saved using GBK.

[fmnijk]

Maybe there should be some warnings like these.

logging.warning("Bone '%s' cannot be decoded using Shift JIS. Using UTF-8 instead.", name)
logging.warning("Bone '%s' cannot be encoded using Shift JIS. Using UTF-8 instead.", string)