Theme Customizer: Improve image picker control. see #19910.

Overhauled image pickers:
* Add support for drag and drop uploads to image controls.
* Improve the 'uploaded' tab in image controls: automatically add images uploaded during the current session, hide the tab when no uploaded images exist.
* Move the header image control to the WP_Customize_Header_Image_Control class. Remove wp_customize_print_uploaded_headers() and wp_customize_print_uploaded_headers() functions.
* Abstract the dropdown functionality from the color picker to the .dropdown class.
* In wp.Uploader, unset element keys if passed an empty jQuery collection.



git-svn-id: https://core.svn.wordpress.org/trunk@20545 1a063a9b-81f0-0310-95a4-ce76da25c4cd