feat: suport systemd type=Notify ordering

[waltmck]

Makes Wayle signal NOTIFY_SOCKET after all service constructors finish. In particular, this (combined with tweaking the .service file to use Type=notify and enforce ordering) prevents xdg-autostart-ed apps from starting too soon and trying to send a notification before Wayle finishes setting up its notification service. Previously, this race could lead to two undesirable outcomes

• Missed notifications
• (with wayle-services #36) A GApplication incorrectly falling back to org.freedesktop.Notifications even when org.gtk.Notifications is available

sd_notify signaling only happens when Wayle is run with wayle shell, not with wayle panel start (this is to prevent unexpected behavior on misconfigured setups that use exec-start=wayle panel shell or similar in a compositor service with Type=notify but without NotifyAccess=main; in this case, Wayle might incorrectly signal readiness for the compositor service it runs in rather than for itself).

Added a new dependency sd-notify, and also needed to fix(?) a build script bug that led to fragile link ordering guarantees.