[UX] New interface for editing, previewing, and saving theme settings

While discussing what to do with the color module in #1905, we took a look at how other services such as Squarespace and Tumblr provide an interface for customizing the appearance of your site and previewing the changes before saving them.

We already provide the ability for themes to add their own custom settings to the theme settings forms. For example Bartik asks why style of tabs it should use for the main navigation menu. However, there is no way to preview the different options without making the changes, saving them, and navigating to a non-admin page that uses the public theme.

Perhaps we should implement an interface just like the ones above that show a bar with the settings on the left and a live preview of the public theme on the right? It could probably be done in contrib to start, but it seems like a good candidate for core functionality, since it would allow us to support a feature that other platforms have already trained users to expect.

We could also include a theme switcher in the sidebar interface, which would satisfy the request for that functionality in #1711.