I made three significant changes in this PR. 

1. `SettingsWindow::fetch_files` now creates
`SettingsUiFile::Project(..)` for any worktree that contains no project
settings.
2. `update_settings_file` now creates an empty settings file if a
worktree doesn't contain one.
3. `open_current_settings_file` also creates a settings file if the
current one doesn't exist.

Release Notes:

- settings ui: Enable editing project settings for worktrees that don't
have a project setting file.