fill layouts from X server when there are no layouts in config
Summary: BUG: 348198
Test Plan:
- move ~/.config/kxkbrc
- connect a USB keyboard
- layout changing works
Reviewers: ngraham, broulik, hein, Plasma
Reviewed By: ngraham
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D21497