Add gimp_image_metadata_update_pixel_size(), _bits_per_sample(),
_resolution() and use them from gimp_image_set_metadata() and from
various places in gimpimage.c which had identical copies of the same
code.

Also add gimp_image_metadata_update_colorspace() which syncs the color
space stored in the image's metadata with the color space of the
image's actual color profile. Call the function from the right places.

The body of gimp_image_metadata_update_colorspace() is currently
disabled because the syncing of color space information is
controversial, see issue ##3532 and issue #301.