Add support for adding widgets

It would be great if the great new widgets UI you're working on could be embedded within in a lightbox window that appears over the window when using the customizer (just like when you open the media manager). Widgets could then be dragged into the relevant customizer sections in the customizer panel (each sidebar could have its own section), and the preview window could then update to show the user what the site looks like with that widget added. 

 - [x] Expand the widget control when it is first added to the sidebar.
 - [x] Prepend the added widget to the existing widgets added to the sidebar. (Need to prepend the `widgets_sidebars` setting.)
 - [x] Is WordPress keeping track of the multi_number outside of the widget form?
 - [x] Don't invoke the widget save option when first adding a widget (to avoid error messages).
 - [x] When removing a single widget, make option to add the widget enabled again.
 - [x] Fix restoring of single widgets (has initial previewing state, and instance override does not work since control looks at `$_POST`)
 - [x] When initially adding a widget, make sure it will appear in the preview even before saving.
 - [x] When adding a single widget which previously existed (and is now in inactive sidebar), the old instance should remain.
 - [x] Refactor all of this to eliminate PHP 5.3 dependencies
 - [x] Remove pre-preview filters once `customize_register` happens
 - [x] Refactor option transactions into separate class.
 - [x] What is wrong with adding the RSS Feed widget?
 - [x] Adding or reusing nonces for security? (e.g. in `preview_new_widgets`)