These default built-in elements are considered superfluous because the
icons themselves can represent folder expansion state.
Icons are intentionally left-aligned so that folder entries share a common
horizontal alignment edge with iconless file items.
Notes
Consideration: do some extension plumbing to only expose a single icon theme
and allow for the icon set variation to be configured via a separate setting.
Consideration: provide a setting that opts into using an empty icon
placeholder for file items. This would horizontally align the text of the
latter with folder items, which might appear clearer for some.
