Add flag for class-level disallow of events, apply to OptionEngine

Fixed bug where events associated with an :class:`Engine`
at the class level would be doubled when the
:meth:`.Engine.execution_options` method were used.  To
achieve this, the semi-private class :class:`.OptionEngine`
no longer accepts events directly at the class level
and will raise an error; the class only propagates class-level
events from its parent :class:`.Engine`.   Instance-level
events continue to work as before.

The comments present another way of doing this where we would
copy events from the parent engine at option time rather
than linking the event listeners, but this would be a behavioral
change that adding new events to the parent engine would not
take effect for an already-created OptionEngine.

Change-Id: Id128516f54103fbad9a2210d6571eceb59c8b0cb
Fixes: #4181

0 files changed, 0 insertions, 0 deletions