Import of all stereo encoded formats (anaglyph, interlace, top-bottom, ...)

This is supported at the moment for single-layer formats only. It works
fine though. The user must be aware that some formats are
highly-destructive and won't never be recovered 1:1 (e.g., interlace and
anaglyph)
Generated Images only give 3d option if scene is Multiview
Image Editor saving of Image in 3d working (it was working for RenderResult images only in the past)
remove ambiguity in if statement