EMSUSD-1521 modify AE section order

- Accumulate all schemas and their attributes before creating any UI for them.
- Re-ordered schemas according to the design.
- Renamed Xformable to Transforms.
- Made all sections be collapsed by default except the prim type section, Light and Light type
- Removed the "Applied Schemas" section.
- Add a unit test and fix some tests that assumed sections would be
  expanded.
- Make all sections (except materials, which has a special logic) be
  re-orderable in the future.
- Update the documentation to reflect the changes.

lib/mayaUsd/resources/ae/usdschemabase/Attribute-Editor-Template-Doc.md

@@ -22,26 +22,37 @@ The AE template has two functions to manage supressed attributes: `suppress`
 supresses the given attribute. `suppressArrayAttribute` which supresses all
 array attribute if the option to display is turned off.
 
-The constructor of the AE template calls `buildUI` to fit each attribute in a
-custom control. Afterward it calls the functions `createAppliedSchemasSection`,
-`createCustomExtraAttrs` and `createMetadataSection` to create some UI sections
-that are always present.
-
-The `buildUI` function goes through each USD schema of the USD prim. If the
-schema is one of the "well-known" ones that have a specialized UI section,
-then that section gets created. For example, shader, transform and display each
-have a specialized UI section.
-
-If the schema is not a special one, then `buildUI` retrieves all attributes of
-the schema and calls `createSection` to create the UI section that will contain
-those attributes. `createSection` checks if the attribute is not supressed and
-calls `addControls` to do the UI creation.
-
-The `addControls` function goes through each custom control and ask each of them
-to create the UI for the attributes. Once a custom control accepts to handle the
-attribute, it calls `defineCustom` to register the custom control with Maya.
-Afterward the attributes are added to the list of already handled attributes.
-The AE template can then proceed to the next schema.
+The constructor of the AE template calls a series of `find`-ing functions to
+fit each attribute in a custom control or other special built-in sections.
+Afterward it calls `orderSections` to choose the order in which the sections
+will be added to the AE template. It then calls `createSchemasSections` to
+create the section UI.
+
+Each `find` function goes through the USD schemas of the USD prim. Some of
+the `find` functinare looking for specific schemas. Other are looking for
+applied schema or class schemas. A few are not looking at schemas at all
+but are grabbing specific kind of attributes. Here are the `find` functions:
+
+- findAppliedSchemas: find the attributes belonging to applied schemas.
+- findClassSchemasL find the attributes belonging to the prim class schemas.
+- findSpecialSections: find the shader, transforms and display attributes.
+
+The `orderSections` function has a list of section it wishes to place first and
+a list of sections it wishes to place last. It orders all the sections that were
+found according to those lists. All remaining sections that are not forced to be
+in a particular order are placed in between.
+
+The `createSchemasSections` function takes the ordered sections and call the
+correct function for each. There is a generic `createSection` to create the UI
+for normal sections. There are a few special `create` functions for the special
+sections, like shader, material, transforms, metadata, etc.
+
+The `createSection` function checks if the attribute is not supressed and calls
+`addControls` to do the UI creation. The `addControls` function goes through each
+custom control creator and ask each of them to create the UI for the attributes.
+Once a custom control accepts to handle the attribute, it calls `defineCustom`
+to register the custom control with Maya. Afterward the attributes are added to
+the list of already handled attributes.
 
 ## Custom Controls