Support chain-select elements in .setting.php files

Overview

As we move to metadata-based settings, we need to support existing use cases such as chain-select.

Example use-case

1. Go to Administer » Localization » Languages, Currencies, Locations.
2. The defaultContactStateProvince setting can't be represented accurately via metadata.

Current behaviour

No way to define either an onclick value or a chain-select.

Proposed behaviour

A new property chain_select_settings contains properties that are passed to the addChainSelect method.

Comments

There's partial support, and I suspect work on this stopped because addChainSelect has a syntax that's tangled up in Smarty. I'll submit a PR to be a conversation piece.

This will also need documentation.