File handles for removed resources not released

[meh2481]

• Open a level
• Delete the only scenelayer with an image
• Clean resources
• Save
• Enter the File Manager and attempt to delete the image
• Note that the image cannot be deleted because the file is in use

The editor should ideally release all file handles after reading in the images, since there's nothing really to keep using them for. It's probably ok to keep the file handle open for images that are active in the level, but if a file is removed from the level there should be no reason to keep the file handle open.