Investigate automatic compositor startup detection via a separate unit

[Vladimir-csp]

Following discussion in YaLTeR/niri#254

Generic compositor readiness detection via a separate unit (Type=oneshot, BindsTo=wayland-session@%i.target, Before=wayland-session@%i.target). A process in that would wait for WAYLAND_DISPLAY to appear, then maybe put some extra vars and exit. This will delay graphical-session.target until WAYLAND_DISPLAY is set.

Pros:

• No requirement for including uwsm finalize in compositor startup if compositor interacts with systemd and activation environments by itself.
• No need to disable compositor's systemd and activation environments interaction.
• Prevents premature downstream units startup if compositor interacts with systemd and activation environments by itself.

Cons:

• Custom variables will miss cleanup lists.
• Compositors that interact with activation environments usually do not bother with dbus quirks.
• Not all compositors can/use Type=notify on their own

A delayer unit to wait for WAYLAND_DISPLAY might be worth adding regardless.
Cleanup can be determined by somehow comparing environment before and after wayland-wm@.service startup.