Better Maya Integration test integration

[Latios96]

UI:

• automatically expand the suite of the currently open test file (if any)
• Provide a Toolbar with icon buttons (Create new Suite, Create new Test, | Divider| Save )
• provide an open button on the currently selected test
• use a folder icon for a suite in the tree view
• display a placeholder image for a test with no thumbnail

Features:

• support creating new tests using a base scene workflow. In this case, only the the corresponding .usda file is created. This should also be possible in a standalone mode
• display if a suite uses a base scene workflow
• automatically register a tests in the config file. Since it is not known which fields a test needs, provide a "test template" on the suite / config level
• support base scene workflow as a first class citizen. It should be possible to create and edit base scenes as it can be done for normal tests. Store it as a Optional[BaseScene] on the suite
• use a better architecture with a class for each layer (UI, logic, dcc handling, storage)
• shorthand to duplicate test
• rename test / suite
• UsdStage integration with layering (reference base scene as a layer)
• fixture library (sphere, cube, suzanne, signle shaderball, multiple shaderballs for parameter exploration) etc. which all get referenced in via USD