Migrate from LightDM for Budgie and Xfce #8731

Detailed Description

Deprecate the LightDM packages and replace them with SDDM. This will involve changes to packages, as well as ISO image package lists. It will also require a theme. Part of this proposal is to package and use the Slick SDDM theme by UbuntuBudgie on Budgie and Xfce systems.

Rationale

LightDM has been languishing for quite some time now. Pull requests have piled up, with no one reviewing or merging them. It has been especially difficult for Nvidia users, because some interaction between LightDM and the graphics drivers causes problems displaying a GUI.

SDDM, while also not free of maintainability issues, does at least see more active development. It makes more sense at this point to swap LightDM for SDDM.

Scope

• Desktop Maintainers: Package slick-sddm-theme and add it as a dependency of the budgie-desktop-branding and xfce-desktop-branding packages. The SDDM and plasma-workspace packages may need modifications to enable setting this theme as the used SDDM theme. Add the replaces key to the SDDM package to replace LightDM on existing installations. Deprecate LightDM, slick-greeter, and related packages.

Upgrade/compatibility impact

If a package upgrade is completed and removes LightDM, it is unclear at this time what will happen if the user logs out before rebooting the system.

If a user with both LightDM and plasma-workspace, which would be the case if they have KDE Plasma installed alongside Budgie or Xfce, we have to figure out which SDDM theme is used, and how that is chosen. While installing KDE Plasma next to another desktop is unsupported by us, users may have done it anyway.