feat/interactive catalog

[tconbeer]

Closes #491
Closes #213

What are the key elements of this solution?

• Adapter Catalogs can now contain InteractiveCatalogItems, in addition to plain CatalogItems. These new classes both support lazy-loading via a fetch_children() method and interactions defined as class variables.
• The HarlequinDriver provides a simple interface for Adapters to take actions in the app
• The DataCatalog has been totally refactored to load its nodes in the background
• The Word and Member completers have been refactored to accept extensions to their catalogs after they are created.
• The Catalog is no longer cached (this may change again in the future).

Why did you design your solution this way? Did you assess any alternatives? Are there tradeoffs?

• Wanted an easy-ish API for adapter authors to implement both lazy loading and interactions
• In the future I'd like to improve on the Completers (unloaded catalog items won't be available to the completers; it should be possible to cache completions but invalidation is hard for deleted objects)

Does this PR require a change to Harlequin's docs?

• No.
• Yes, and I have opened a PR at tconbeer/harlequin-web.
• Yes; I haven't opened a PR, but the gist of the change is: need to update the contributing guide for adapter authors at some point, probably with a new page for the catalog.

Did you add or update tests for this change?

• Yes.
• No, I believe tests aren't necessary.
• No, I need help with testing this change.

Please complete the following checklist:

• I have added an entry to CHANGELOG.md, under the [Unreleased] section heading. That entry references the issue closed by this PR.
• I acknowledge Harlequin's MIT license. I do not own my contribution.