plasma5: set default session to plasma X11 and fix sddm

[oxalica]

Motivation for this change

As suggested by @ttuegel in #139657 (comment) and many others in #143272, set the default session to plasma X11 instead of wayland for now.

sddm upstream currently doesn't support system-wide default session (sddm/sddm#326), I implemented it in a patch in this PR. I'll maybe upstream it later if time permits.
Note that if you logined non-default session once, it will be selected in the next time. That is, the last logined session still takes precedence over system-wide default session.

Tested in nixos vm.

cc: @abbradar @tobiasBora

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[oxalica]

Result of nixpkgs-review run on x86_64-linux 1

2 packages marked as broken and skipped:

• libsForQt512.sddm
• libsForQt514.sddm

1 package failed to build:

• nixos-install-tools

1 package built:

• sddm (libsForQt5.sddm)

Kind of strange that nixos-install-tools failed. I can successfully build it with nix build -f . nixos-install-tools without problem.

[SuperSandro2000]

someone who knows cpp please look at the patch

[ttuegel]

Thanks!