Theme Customizer: Properly bind the 'upload new' and 'remove image' actions in...

Theme Customizer: Properly bind the 'upload new' and 'remove image' actions in the image picker. see #19910.

This occurred because we weren't calling UploadControl.ready (where the actions are declared and the uploader is initialized) in ImageControl.ready.

git-svn-id: https://core.svn.wordpress.org/trunk@20301 1a063a9b-81f0-0310-95a4-ce76da25c4cd