All great suggestions - thank you.
There is already a small indication when there are unsaved changes. The name of your project at the top of the canvas view should have a little * on the end of it when there are pending unsaved changes - this will disappear when there are none. So if you want to be sure that CTRL + S has worked, that should be a good indicator.
I do agree that it could be more obvious, so I can take a look at adding a small popup or something.
Thanks again!