Depend on lots of newer library versions

Gegl >= 0.1.8, Babl >= 0.1.6, Gdk-Pixbuf >= 1.24.0, Pango >= 1.29.4,
GLib >= 2.28.8, GTK+ 2.24.7

Which means depending on a gazillion of bug fixes, which means less
pain for GIMP 2.8 users, and less useless bugzilla traffic eating
developer resources.