The image which source is set to file is not expected to have empty
file path. If it happens it becomes very tricky to save the image on
exit using the standard quit dialog.
This change makes it so if the image buffer does not have file path
then the new image is set to the "generated" source and it behaves
as if the image was created like so and was fully painted on.
Additionally, mark image as dirty, so that quitting Blender after
such image was added will warn about possible data loss.
c922b9e2c1