- 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
It was misnamed from the beginning and has no relation to GimpImageMap
except that it happens to be used by GimpImageMapTools. Now it feels
less weird to potentially use it for other settings too.