because it confuses gtk-doc and breaks some links. Also change the
"Index of new symbols in GIMP 2.x" sections to be what seems to be the
modern standard (looked at the GLib and GTK+ docs), and update some
other stuff.
- 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
