User Settings: Implement settings components

[tjcouch-sil]

As an extension developer, I want to be able to expose my own user settings on the PAPI synchronously so users can configure them and I and others can use my settings.

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Add a way for extensions to register a component in activation for each setting they want to be able to display in the User Settings Dialog.

Depends on #502. Tightly coupled with #503. See #714 similar