Fix saving files in scene sub-folders

[manongjohn]

This fixes some saving issues related to saving when Project Management -> Project Settings -> Separate assets into scene sub-folders is enabled.

1. Fixes when importing images into an untitled (unsaved) scene, saving and then reloading resulted in image loss.
2. Fixes an issue when saving images drawn in an untitled scene the files are saved in Untitledxxx folders in +extras and +drawings instead of the scene name.
3. Fixes an issue when saving a scene with Save As to give it a new name, prior images are still referenced in the old scene subfolder. Now it copies files to the new scene subfolder.