display-manager: Switch to plasma-login-manager

The ISO now uses plasma-login-manager, which uses kwin-wayland as the compositor under the hood. Motivation: there have been numerous reports of users unable to enter the Live ISO due to a Qt/SDDM crash. The default desktop session for the ISO is also changed to Wayland. Known issue: - Calamares does not update the keyboard layout properly on Wayland and needs a pkexec wrapper to pass the environment so it inherits theming. TODO: - [x] Implement a pkexec wrapper that passes the environment correctly to pkexec so Calamares follows our theming. - [x] Calamares change keyboard.cfg for "kwin" support at locale1 - [ ] Testing.