26147 mari polish

- Addressed code review comments from previous pull request.
- Loader actions are suppressed when creating a new project.  Previously it was possible to perform actions defined for the loader when creating a new project in Mari - this has been disabled.
- Double-clicking on a publish in the open-publish dialog now performs the open action.
wondering if this is a bit hacky? Maybe the action manager should always be passed in via the constructor arg and then created here: https://github.com/shotgunsoftware/tk-multi-loader2/blob/master/python/tk_multi_loader/__init__.py#L33 ?