GimpOperationPosterize: also posterize the alpha channel. Seems like a
no-brainer to me, and is actually useful (see bug). The old behavior
is still available by simply disabling the alpha channel in the
channels dialog.
- 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
