PaYaml schema and OM updates

[joem-msft]

Update schema and OM according to latest changes

Schema Updates:

• Update to use Control Type Identifiers in schema
• Added Groups on Screens, ControlInstances, ComponentDefinitions per XR
• Added CodeComponent contorl instances, aka PCF controls, per XR
• Added VSCode snippets for App, Screens, and ControlInstances
• Removed App.Children.Host from schema; it needs additional design review

Model Updates:

• Update ordering of PaFileRoot properties
• Add tests which verify when full usage of schema is used that round-trip is not lossy
• Added support for Groups on Screens, ControlInstances, ComponentDefinitions per XR
• Added extension method DescendantControlInstances for iterating thru control tree in document order
• Added unit tests for DescendantControlInstances
• Remove AppInstance.Children as the current design is not sufficient