This change defines additional theme contexts for editor
branches to prevent theme leaking between the default
theme, the project theme, and the editor theme.
- Both editor window and EditorNode define an editor-specific
context with the editor theme and the default theme.
- The 2D viewport defines a project-specific context with
the project theme and the default theme.
- Theme editor preview tabs define the default-only context
with the default theme.
Additionally, the default theme context now only includes
the project theme for running projects (both export and debug).
This prevents the project theme from leaking into the editor.
This commit also does a little clean up on the theming aspects
of the EditorNode.
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