Features are flags, therefore we must NEVER use them without a shift,
otherwise bad mistakes happen, like the previous mistake of declaring
WP_SESSION_FEATURE_LINKS as the number after WP_SESSION_FEATURE_ENDPOINTS,
which ended up being (WP_SESSION_FEATURE_ENDPOINTS | WP_PROXY_FEATURE_PW_PROXY)
and it was always becoming available together with the ENDPOINTS feature.

This aligns better with the general design of consuming property
objects on constructors, both in PipeWire and WirePlumber APIs

It can be a bit tricky because the "installed" signal may be fired
from inside the context of wp_core_install_object_manager(),
in which case the main loop should not be executed (or it will never quit)

We can now call wp_proxy_request_destroy() on endpoint links and
the WpImplEndpointLink together with the session item that created
it will be cleaned up

+ better logs by default
+ allow loading modules in tests

+ expose the export transition in the session item class
+ make the export-related flags immutable
+ add an export error flag
+ update and improve documentation

+ replace calling execute_step(..., STEP_ERROR) with rollback
+ implement deactivate internally using rollback

This unifies deactivation steps, which are common between deactivate()
and calling execute_step() with WP_TRANSITION_STEP_ERROR at the
end of a failed activation transition.

including wp/spa-props.c currently re-defines G_LOG_DOMAIN
in the spa-props.c unit test

+ enable the new log writer on the executables
+ enable structured logging in the tests