Switch from slim to lightdm as the display-manager.
If plasma5 is used as desktop-manager use sdddm.
If gnome3 is used as desktop-manager use gdm.
Based on #12516
The wallpaper used is *structurally compatible* with the other one,
meaning that the logo is at the same location, and not bigger.
It has one drawback: the logo is brighter, which clashes with the grub
usage. This is to be fixed with new options in grub.
The default session might be found in `extraSessionFilePackages`, but it's not
viable to detect at evaluation time, so emit a warning.
In LightDM instead of checking `defaultSessionName` against
`displayManager.session.names` we rely on the assertions in
`desktopManager` and `windowMananger` and just check that there's at least one
default set. The second assertion could never actually be triggered.
This makes it easier to support a wider variety of .desktop session files. In
particular this makes it possible to use both the «legacy» sessions and upstream
session files.
We separate `xsession` into two parts, `xsessionWrapper` and `xsession`.
`xsessionWrapper` sets up the correct environment and then lauches the session's
Exec command (from the .desktop file), falling back to launching the default
window/desktopManager through the `xsession` script (required by at least some
nixos tests).
`xsession` then _only_ handles launching desktop-managers/window-managers defined
through `services.xserver.desktopManager.session`.
Pass gnome-session to extraSessionFilePackages, remove unnecessary environment variables, move the rest out of old session option, and then drop the option.
Previously, the mkDesktops function produced a flat package containing
session files in the top level. As a preparation for introduction of
Wayland sessions, the files will now be placed to $out/share/xsessions.
To update the plasma start menu `kbuildsyscoca5` needs to be executed.
There are several people complaining about missing applications in their
plasma start menu.
This patch adds a activationScript for plasma, that runs
`kbuildsyscoca5` for each user that has `isNormalUser` == `true`.
