app,libgimp*: fix includes to follow our include policy

- 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