The bug occurred because `cx.observe_new::<Workspace>` would pass a
mutable `Window` that was also the new workspace's window. Later,
`fetch_files` would fail to read the newly created workspace from
`WorkspaceStore` because its window already had a mutable reference to
it.

The fix always passes the Settings UI's window handle to the
`fetch_files` function, and uses a `settings_window_handle.defer`
callback to call `fetch_files` after the newly created workspace's
window no longer has any mutable references to it.

Release Notes:

- settings_ui: Fixed editable project settings not being updated when a
new workspace is created