Systemd Startup

Summary:
This brings numerous advantages such as:

- easier admin configuration with drop-ins, overrides and multiple
hooks throughout (and only having to learn one tool)

- session cleanup on exit, avoiding that occasional part where shutdown
hangs

 - startup that actually knows when things are up

 - race free autostart and DBus activation at once

 - logs that rotate are split by service and usable

 - resource management through slices and cgroups (the part I want)

Over the past 2 years I've been trying to tidy up and encapsulate the
relevant parts of startup into the binary plasma-session so that we can
just runtime swap out that one part and supporting both paths will be
easy.

Support is toggleable via cmake flag, as it seems like it should be a
distro decision, especially as we will require a specific systemd with
the xdg-generator.

KDED/kwin/other services are attached to the relevant repo.

Task T11914

Differential Revision: https://phabricator.kde.org/D28305

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