This is more explicit as for why this functionality isn't available
depending on editor settings and current platform.
This also exposes a `EditorInterface.is_multi_window_enabled()` method
so that editor plugins can easily query whether the editor is able and
expected to create multiple windows.
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.
