Summary: Can't change GTK3 theme in lxqt-config-appearance if settings don't exist, so create it.
Test Plan:
1. Open lxqt-config-appearance
1. Check "Set GTK themes"
1. Change "GTK 3 Theme" to Arc
1. Close lxqt-config-appearance
1. Open lxqt-config-appearance
"GTK 3 Theme" should be set to Arc and not the default, Adwaita.
Reviewers: tsimonq2
Reviewed By: tsimonq2
Differential Revision: https://phab.lubuntu.me/D13