BUG:310359 Fix loading and saving Lab TIFF images

Internally, Krita uses the ICCLAB definition, not the CIELAB
definition, so we shouldn't convert what we read to CIELAB, and
should save what we save tagged as ICCLAB.

Photoshop CS2 and CC can now load our TIFF Lab images! (We don't have
a gimp devel build to test with atm.)