sddm-theme: Populate keyboard layouts menu only on first show
Doing it synchronously can be very slow; seemingly scaling with the number of items in the model. https://bugzilla.redhat.com/show_bug.cgi?id=2179998
Doing it synchronously can be very slow; seemingly scaling with the number of items in the model. https://bugzilla.redhat.com/show_bug.cgi?id=2179998
mentioned in commit 45d09683