Updated CodePageIdentifiers

[fvanroie]

• Added new codepages: Shift-JS and UTF-8.
• Swapped to byte instead of ushort. The other byte is a multi-byte flag and can be derived from the Windows Encoding flags.
• Decorated the enum with descriptions of the Windows Encoding name, so an easy mapping can be made between CodePageId and Windows Encoding.

Note: Swapping from byte to ushort has an impact in read/save routines. I'll address those later.