Support #include and #base directives in SerialisedObject

[LogicAndTrick]

See valve implementation of these directives:

https://github.com/ValveSoftware/source-sdk-2013/blob/0d8dceea4310fde5706b3ce1c70609d72a38efdf/sp/src/tier1/KeyValues.cpp#L2110

https://github.com/ValveSoftware/source-sdk-2013/blob/0d8dceea4310fde5706b3ce1c70609d72a38efdf/sp/src/tier1/KeyValues.cpp#L2126

Valve also has "conditionals" which is hard-coded to a list of platforms. Don't really think this needs to be supported, but currently it's not.

https://github.com/ValveSoftware/source-sdk-2013/blob/0d8dceea4310fde5706b3ce1c70609d72a38efdf/sp/src/tier1/KeyValues.cpp#L2055-L2086