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.
Do we support PTEX in toolkit core (in the templates system?) - am I right thinking it's currently only supporting UDIM? Is this worth mentioning here in a comment?