Issue description

When opening the file picker of a matplotlib (gtk3 backend) window the process crashes with this error:
GLib-GIO-ERROR **: No GSettings schemas are installed on the system
I think it needs wrapGAppsHook but I'm not really sure how that works.

Steps to reproduce

$ nix-shell -p 'pythonPackages.matplotlib.override {enableGtk3=true;}' --run 'python -c "import matplotlib.pyplot as plt; plt.plot([1,2,3],[4,5,6]); plt.show()"'

Technical details

NixOS version: 17.09pre106117.c90998d5cf