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.
- don't include <gdk-pixbuf/gdk-pixbuf.h> in headers in app/
- instead, include it in many .c files instead of <glib-object.h>,
finally acknowledging the fact that app/ depends on gdk-pixbuf almost
globally
- fix up includes as if libgimpbase depended in GIO, which it soon will