Use more notifications

[berombau]

Napari has support for notifications/flash messages/toast popups, but the API seems internal for now.

See docs and examples at:

• https://napari.org/stable/api/napari.utils.notifications.html
• https://napari.org/stable/plugins/first_plugin.html#add-plugin-functionality-to-napari-hello-init-py

Places where we can implement this:

• [x] Start + End Info message for clean/segment... step if it takes >5 seconds
• [x] If output files are written, show the output location in and Info message, preferably with a clickable link
• [x] Warning if e.g. CPU segmentation is used and it will take a really long time
• [x] Do not use extra error notifications, only if the regular error is not clear enough for the user to correct their action