Improvements around saving annotations to disk

[LucaMarconato]

As discussed with @melonora, currently we don't save napari annotations to disk when Shift+E is pressed, we should re-enabled it (this was automatically done before the incremental IO refactoring, but now the data is only updated in-memory).

Also, a related issues is:

• the tooltips, in particular the one appearing with Shift+E, is not shown when napari is run in interactive mode. This issue is already open here: show_info() not working when running napari from a Jupyter notebook napari/napari#6455. A user found this confusing as he was not clear if the data was saved or not.