It's not well-defined; we'll come back to that later

- Use similar code for consistency
- Add changed signals everywhere
- Port to the new object-manager API

signal handlers expect FEATURE_BOUND to be set

This way we can more easily distinguish different objects when
looking at the log, although it's not perfect

Add the 'const' attribute to let the compiler know that it doesn't
need to call it multiple times for the same debug level argument,
since the enabled log levels cannot change at runtime.

The 'installed' signal can be used to know that there are no
known objects that are being prepared internally, so the object
manager is ready to use.

This also improves internal state management so that the 'objects-changed'
signal cannot be fired earlier than it should. Previously there
were corner cases with complex proxy features, as the object manager
relied on the fact that after a core 'sync' it is safe to assume
that all proxies are augmented... that's not always the case.

+ add a ports-changed signal

Implementations are not required to provide implementations for these
if they don't need them

+ add the useful _find_proxy() method

+ 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.