Closes #ISSUE
This PR includes the necessary work to get `EditorSettings` showing up
in the settings UI. Including making the `path` field on
`SettingsUiItem`'s optional so that top level items such as
`EditorSettings` which have `Settings::KEY = None` (i.e. are treated
like `serde(flatten)`) have their paths computed correctly for JSON
reading/updating.
It includes the first examples of a pattern I expect to continue with
the `SettingsUi` work with respect to settings reorganization, that
being adding missing defaults, and adding explicit values (or aliases)
to settings which previously relied on `null` being a value for optional
fields.
Release Notes:
- N/A *or* Added/Fixed/Improved ...
54cec5b484