Change legend "saving" to "generating thumbnail"

When I close the Inspector for my scene, I then see the text "saving" over my scene's tile.
This is misleading, because the scene is actually already saved. A user might think that they can't close the app while it's in this state.
The reality is that at this point we're just generating a new thumbnail (I think, I could be wrong).


We should then change the text to something more accurate
"Generating Thumbnail"