Godot 3.2: Add ClassType wrapper and SceneTemplates

[willnationsdev]

SceneTemplates are like editor-only script classes for scenes (they show up in the CreateDialog). ClassType is a convenience wrapper around engine classes, script classes, and scene templates.

[willnationsdev]

I am now in the process of refactoring this branch in the following ways:

• Make CustomTypes indexed by name rather than by base type, to make it more like script classes and make tracking inheritance hierarchies easier.
• Make CustomTypes support scenes as well.
• Create a tab in the ProjectSettings for viewing and editing CustomType records.
• Modify EditorPlugin.add_custom_type to interact with the new ProjectSettings tabs' data.
• Add editor context menu shortcuts
  • FileSystem dock
    • Script file: "Promote to Custom Type Script"
    • Scene file: "Promote to Custom Type Scene"
  • Scene dock
    • Node with a script: "Promote to Custom Type Script"
    • Root node of scene: "Promote to Custom Type Scene"
• Create an abstraction layer class around all class data, "ClassType". It will be a Reference type that provides a ClassDB-like interface that spans all class types:
  • engine classes
  • anonymous scripts
  • anonymous scenes
  • Custom Type scripts
  • Custom Type scenes
  • Script Class scripts
• Update the CreateDialog to use the ClassType object.

[willnationsdev]

Based on the discussion in godotengine/godot#21187, there is no need to continue working on this. Anything that can assist both the editor (can't be a module, must be in core or editor) and runtime (can't be in editor) would have to be in core, and any and all core changes that would enable a user-defined type system will not be approved by reduz.