kcms/wallpaper: fix localization

Messages.sh was missing, and the domain was set incorrectly for some strings.

Amends 779e2d07

BUG: 505122
BUG: 508491
FIXED-IN: 6.5.0

I'm assuming cherry-picking is fine since this fixes a localization bug, and doesn't change any strings that have already been localized. But please correct me if that's wrong.