PaYaml schema and OM updates (#651)

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
- Rename `ControlInstance.ControlType` to `ControlTypeId` to keep consistency
- 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
- Renamed `PersistenceException` to `PaPersistenceException` to remove naming ambiguity when in doc svr code

Also:
- Added build script that will process schema files. It removes comments and also enables some build-time modifications to the schema for public consumption.

.vscode/settings.json

@@ -1,13 +1,22 @@
 {
   "cSpell.words": [
     "Dataverse",
+    "msapp",
     "PPUX",
-    "msapp"
+    "RGBA"
   ],
   "[yaml]": {},
   "yaml.schemaStore.enable": true,
   "yaml.validate": true,
-  "yaml.schemas": {           
-    "https://raw.githubusercontent.com/microsoft/PowerApps-Tooling/master/docs/pa.yaml-schema.json": "*.pa.yaml" 
-  },
+  "yaml.schemas": {
+    "http://json-schema.org/draft-07/schema#": "*.schema.yaml",
+    "src/schemas/pa-yaml/v3.0/pa.schema.yaml": [
+      //"*.pa.yaml",
+      "src/schemas-tests/pa-yaml/v3.0/**/*.pa.yaml"
+    ],
+    "https://raw.githubusercontent.com/microsoft/PowerApps-Tooling/master/schemas/pa-yaml/v3.0/pa.schema.yaml": [
+      "src/schemas-tests/**/*.pa.yaml"
+    ],
+    "https://raw.githubusercontent.com/microsoft/PowerApps-Tooling/master/docs/pa.yaml-schema.json": "src/Persistence.Tests/**/*.pa.yaml"
+  }
 }