Re-add Dialog feature

[PoignardAzur]

Dialogs, especially platform-created file pickers, are a feature from Druid that was ripped out when I wrote Masonry and haven't added back in yet.

Part of the code is still in the codebase, commented out, taunting me.

Ideally, I would want the Masonry implementation to use the promise feature, but this is a more complicated task than it might appear.