Implement ::observeScopedKeyPath using ::onDidChangeScopedKeyPath
Handle null key-path in Config::set
Deprecate Config::addScopedProperties
Handle a source but no selector in ::unset
Add editor.completions to config schema

It was implicitly part of the schema already, but it used
to be set in a way that bypassed schema validations
Merge pull request #4773 from atom/mb-remove-add-scoped-properties

Deprecate `Config::addScopedProperties`