(REF) AuthorizeEvent - Extract AuthorizedTrait

(REF) AuthorizeEvent - Extract AuthorizedTrait

The primary purpose of this is to provide a trait (`AuthorizedTrait`) to
describe the common semantics of of coarse-grained authorization check and
the upcoming fine-grained authorization check.

The extracted trait makes a few small changes:

* Change the default value from `FALSE` to `NULL`.  In grepping universe for
  consumers of `isAuthorized(0`, I could only find consumers that used
  bool-ish values.  So this should be the same for them.  However, for
  future cases, it will allow some distinction between NULL/FALSE.

* Use more type-hints. The type should be nullable-boolean.

* Mutators should be amenable to fluent style (e.g. `$event->authorize()->stopPropagation()`).