We check them into git, so this makes it easier to keep them in
sync when using a separate build directory.
Case in point -- this commit also syncs a few enum files that went
out-of-sync with their headers.
- 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
