JPEG 2000 codestream (.j2k/.j2c) are only compressed code stream data,
without header. In particular we don't have color information, such as
the color space. So we need to open a dialog asking to set the color
space in interactive mode.

Note: according to OpenJPEG developers, a JP2 image (not codestream)
should always have a color space defined in its header. But just to be
flexible, the same dialog may get raised as well if we try to load a JP2
with no valid color space defined in header and no ICC profile embedded.
Maybe if such a thing happened, it means the image is corrupt, yet we
may as well try and salvage it anyway.

Note 2: I also removed a weird test which was setting some images as
being YUV color space by mistake. This actually fixes bug 794413 as a
side effect.