Turberfield-dialogue

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* `Stateful.set_state` now takes multiple positional arguments.

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* Fix a bug in creating a Persona from an Assembly

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* The `fx` declaration now has a `label` option. A label may may contain
substitution references.
* `Still` cues now have a label attribute which takes its value from the `fx`
declaration. The main use case for this is to provide content for the `alt`
attribute of an HTML `img` tag.

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* `SceneScript.Folder` interludes may be `None`.

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* The `fx` declaration now generates Audio and Stills
* Added documentation for the `Matcher` class.
* Added guidance on alternative for file suffix.

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* Added a Matcher class which can select folders by their metadata.
* `rehearse` function uses the matcher to branch to different folders.
* `turberfield-dialogue` utility uses the matcher likewise.
* **Interludes from now on must return a metadata dictionary**. Fixed the
documentation and demo scenarios accordingly.
* Fixed a bug affecting the TerminalHandler when *simpleaudio* is not available.
* Simplified the documentation relating to VSCode.