app: allow core file procedure which don't return an image.
This is useful to be able to support file formats other than image formats. In particular I will use this in the next commit to support a "GIMP extension" format. When GIMP will open such file, it will install an extension (not open an image on canvas). This is an internal flag only, i.e. only usable from core GIMP. File formats which a plug-in can register are still only image file formats.
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