Add JS Widgets as Shortcodes (Post Elements) with Shortcode UI (via Shortcake)

[westonruter]

• Ensure that \WP_JS_Widget::$icon_name gets exported to an inline style rules to apply to the widgets listed in the customizer.
• Factor out the Shortcake-specific PHP logic from the main Plugin class into another Shortcake-specific integration class.
• Pages widget dropdown fails to parse/restore previously-selected exclusions in Select2 dropdown.
• Ensure that widget can be previewed via shortcake and drafted in post content. (Irrelevant since the widget instance data is inline.)
• Let widget instance data be encoded as JSON in a shortcode attribute.
• Re-use the dashicon associated with a given widget in the customizer to also be used for the Post Element browser in the editor:
  
• Ensure that base styles for widgets are included in widget-form.css so that elements will appear as expected, and not like this:
  
• Update Nav Menu widget's Edit Menu button to work as expected, to be hidden when no menu is selected and to work properly in a Shortcake context.

Fixes #11.

[westonruter]

This is ready for final testing and merging now.