Theme Customizer: Add 'choose image' functionality to image controls. Rough...

Theme Customizer: Add 'choose image' functionality to image controls. Rough first pass, using header images as an example. see #19910.

git-svn-id: https://core.svn.wordpress.org/trunk@20290 1a063a9b-81f0-0310-95a4-ce76da25c4cd