Can't add custom actions to dialogs

Steps to reproduce

1.  Create an Action which extends from a class other than CancelDialogAction or SaveDialogAction (both deprecated)
2.  Add such action to a dialog on the PostCreate method (as stated on the documentation)

Expected results

The action appears at the bottom of the dialog.

Actual results

If the action doesn't extend from SaveDialogAction or CancelDialogAction, no action appears.

Workaround

Development notes

An option would be to expose ACTION_CONVERTERS same way as FIELD_DEFINITION_CONVERTERS.