Theme Customizer: First pass at image controls. Use header_image as the...

Theme Customizer: First pass at image controls. Use header_image as the initial case. Add a 'removed' control param for upload/image controls to map 'removed' to a value other than the empty string. see #19910.

git-svn-id: https://core.svn.wordpress.org/trunk@20278 1a063a9b-81f0-0310-95a4-ce76da25c4cd