Adds a bool import option `nodes/import_as_skeleton_bones`.
This is supported in all FBX or GLTF document based formats.
It is especially useful for retargeting and importing animations.
This change introduces a new EditorThemeManager class
to abstract theme generatio and its subroutines.
Logic related to EditorTheme, EditorColorMap, and editor
icons has been extracted into their respective files with
includes cleaned up.
All related files have been moved to a separate folder to
better scope them in the project. This includes relevant
generated files as well.
