sway: respect systemdSupport and dbusSupport (#160972)

Sway can be compiled with or without systemd(-logind) and dbus.  This
commit exposes that support via the global systemdSupport and
dbusSupport arguments, which are understood by many other nixpkgs
expressions and can be set globally in ~/.config/nixpkgs/config.nix.

This commit also adds a third argument, trayEnabled, which allows to
disable sway's tray.  The tray requires dbusSupport and
systemdSupport.

Reviewers of this commit asked for potential use cases.  There are
many of them; a very non-exhaustive list includes:

* Use of nixpkgs on operating systems which systemd does not support,
  such as MacOS/Darwin, FreeBSD, OpenBSD, or Alpine Linux.

* Use of nixpkgs on *-musl platforms, which systemd does not
  officially support (out-of-tree patches to support musl exist for a
  few systemd versions).

* Use of sway in situations where dbus is inappropriate, such as
  sway's "kiosk mode".

* High-security environments, where the additional attack surface
  exposed by dbus outweighs any features it may offer.

This is a very non-exhaustive list.

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