Customize: Fix interface alignment between `Setting` and `Control`, adding...

Customize: Fix interface alignment between `Setting` and `Control`, adding `defaults` to `wp.customize.Setting` and using `wp.customize.previewer` as default `previewer` param.

Also move jsdoc from class to `initialize` method and correct the param types.

Amends [41726], [42037], [32681].
See #42083, #30737.

Built from https://develop.svn.wordpress.org/trunk@42038


git-svn-id: https://core.svn.wordpress.org/trunk@41872 1a063a9b-81f0-0310-95a4-ce76da25c4cd