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

app/display/display-enums.c

@@ -2,7 +2,7 @@
 /* Generated data (by gimp-mkenums) */
 
 #include "config.h"
-#include <glib-object.h>
+#include <gio/gio.h>
 #include "libgimpbase/gimpbase.h"
 #include "display-enums.h"
 #include"gimp-intl.h"