Bug 735516 - Crop to content fails

Return a new enum GimpAutoShrink { SHRINK, EMPTY, UNSHRINKABLE } from
gimp_pickable_auto_shrink() because a simply boolean FALSE doesn't
distinguish between "not further shrinkable" and "no content". Change
the callers accordingly add a special "already cropped" message where
we only had a "no content" message before.