Balladeer

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* Expose rank when expanding performance commands.
* Allow UI to hide commands by rank.

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* Speech is now gathered from each drama object every turn.
* Expand scope of event target selection to include all drama objects.
* Pin to busker 0.22.0 or newer.
* Accommodate puzzle chain as a list.
* Add `is_command_hidden` method to Performance class.

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* Rename some internal methods.
* Doc fixes.
* Fix a bug where Prologue cues disrupted SpeechTable menus.
* Fix a bug which left tags in SpeechTable branching options.
* Call Stager.terminate method for both terminal and non-terminal states.
* Fix trigger bug in Stager.monitor_context.
* Add transition method to Fruition type.
* Add update method to Entity.
* Test and refactor `StoryStager.monitor_context`.
* Add `Director.express_frustration` method.
* Add option to render in HTML4 (supporting Dillo as a test tool).
* Re-pin to busker 0.15.0.

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* Resident exits are named-tuples.
* Resident class provides `focus` property.
* Multiple fixes to StoryStager.

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* By default, text in `code` emphasis is expanded into a form/button which posts the text as a command.

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* Improved behaviour in StoryStager for populating types.
* Minor improvement to asset path reporting.
* Case conversion on StoryStager state lookup.
* Expect Events from busker.
* New method `monitor_context` to process Events.