Implement text KeyValues 3

[xPaw]

• Flagged values
  • Add Flag to KVValue for flagged strings (text) or flagged anything (binary)
  • Ensure that KV1 serialiser drops the flag when serialising (should happen by default, but check / write unit test)
  • Ensure that flag and value is maintained when deserialising to a strongly-typed object model that has a KVValue field/property.
• Create KV3 text deserializer
• Create KV3 text serializer
• Write tests for converting KV3 to KV1
• Write tests for converting KV1 to KV3
• Support null type
• Support different types of root values (arrays, strings, nulls, etc)
  • This partially works but a few things don't like value flags and arrays.
• Add support for accessing and setting header encoding/format. We will likely need a breaking change for deserialize method to return some kind of KV file object, rather than KVObject directly. This KVFile object will likely be needed for KV2 implementation too.
• Better support for arrays
• Verify whether latest Source2 versions still support multiple flags at once (might have been removed)

[xPaw]

@yaakov-h Can you give this an initial review pass? I especially want to hear about stuff around handling arrays and flags.