fritzing, cura: No GSettings schemas upon file picker dialog

[ralsei]

Issue description

Both Fritzing and Cura fail to open/save files with the same error (which are likely closely related):

(Fritzing:4503): GLib-GIO-ERROR **: No GSettings schemas are installed on the system
(python3:5202): GLib-GIO-ERROR **: No GSettings schemas are installed on the system

They both end in a trace trap and crash. The second one may be more related to Python than Cura. This also only happened after I switched from xmonad to XFCE.

Likely closely related to #24880 and #21647.

Steps to reproduce

Fritzing:

• nix-env -i fritzing
• Run Fritzing
• Attempt to save the current project
• Examine trace trap

Cura:

• nix-env -i cura
• Run Cura
• Attempt to open the file picker and select an STL
• Examine trace trap

Technical details

• System: NixOS 17.09pre102441.45344fd (Hummingbird)
• Nix version: 1.11.7
• Nixpkgs version: 17.09pre102441.45344fd

[Mic92]

Usually this can be fixed the following way: deeddbd

[abbradar]

I plan to fix this and several similar issues later but I'm a bit short on time nowadays; if someone outruns me I won't hold a grudge :D

[abbradar]

cura should already be okay on master since a40b7d0.