Fix network-manager-applet startup issue 

When starting nm-applet, I was always getting

'GLib-GIO-Message: Using the 'memory' GSettings backend.  Your settings will not be saved or shared with other applications.'

To solve this I applied the same changed that were done for virt-manager in [https://github.com/NixOS/nixpkgs/commit/413ebfb2469b394a4f4d48ba3567be3c68344722]