Show list of changes (dirty settings) in Customizer

[westonruter]

Just as we have a UI for showing the settings inside of a snapshot when in the WP admin, this same kind of interface should be available when you are inside the Customizer. There can be a dialog that shows all of the changes, this UI could also provide a way for you to revert an individual change back to the setting's original value. If we mark it as not _dirty, we may need to take care with #60, as you'd not be able to remove a setting from a snapshot as you might expect in that interface, so there may need to be an additional list of setting IDs passed when saving a snapshot for settings that should be reverted (removed from snapshot), just as for concurrency conflict resolution we may need to pass a list of setting IDs that should override any conflicts.

[westonruter]

See also #21666: Customizer reset/undo/revert

[westonruter]

There is now a natural place for this in the Publish Settings section.

Under (or over) the link to discard changes there could be a details element that says how many changes (dirty settings) there are in the changeset:

Expanding that could list out the changes (by control label) where if you click on one it will expand the control's section and focus on the control:

It would be great then to be able to revert a setting as well, but that would be non-trivial in the case of nav menus and widgets. Special logic would be required.

See convo at https://wordpress.slack.com/archives/C0381N237/p1506990600000225