Project properties is rather hard to find. And once you’ve used it, it’s also very hard to hide. I had huge trouble finding it the first time I used v3 - and even harder time closing it.
How about just doing it Adobe way: combine Inspector and Project properties into one panel, and show appropriate UI elements depending on what the user has selected in app. If you have a label selected, show options for label. If you have nothing selected (or maybe a mockup, since “nothing” is reserved for notes), show options for the project.