Fix pre-Initialized ExtEvents not invoking

[combobulator]

In the current commit, the PersistentListener._initializationSuccessful flag is only set when the PersistentListener initializes itself during invocation. The PersistentListener._initializationComplete flag is set properly during the initialization call. Both of these flags must be set for the invocation to execute.

This leads to a failure to invocate, with no error message, if ExtEvent.Initialize() is called before ExtEvent.Invoke(); the contained PersistentListeners will have the _initializationComplete flag set, but _initializationSuccessful not set, in spite of no issues.

This PR sets the _initializationSuccessful flag within PersistentListener.Initialize() so pre-initialized ExtEvents will fire properly.