Introduce new registration methods for custom headers and custom backgrounds. Backwards compatible, but old methods will be deprecated. see #20249. see #17242.

Custom header: Use add_theme_support('custom-header', $args) instead of add_custom_image_header(). Deprecates all use of constants.
 * HEADER_TEXTCOLOR is now (string) 'default-text-color'.
 * NO_HEADER_TEXT is nowi ! (bool) 'header-text'.
 * HEADER_IMAGE_WIDTH (and _HEIGHT) are now (int) 'width' and 'height'.
 * HEADER_IMAGE is now (string) 'default-image'.
 * The 3.4 arguments 'suggested-width' and 'suggested-height' are now just 'width' and 'height' (they are "suggested" when flex-width and flex-height are set).
 * Callback arguments for add_custom_image_header() can now be passed to add_theme_support().

Custom background: Use add_theme_support('custom-background, $args) instead of add_custom_background(). Deprecates all use of constants.
 * BACKGROUND_COLOR is now (string) 'default-color'.
 * BACKGROUND_IMAGE is now (string) 'default-image'.
 * Callback arguments for add_custom_background() can now be passed to add_theme_support().

Inheritance: add_theme_support() arguments for custom headers and custom backgrounds is a first-one-wins situation. This is not an unusual paradigm for theming as a child theme (which is included first) overrides a parent theme.
 * Once an argument is explicitly set, it cannot be overridden. You must hook in earlier and set it first.
 * Any argument that is not explicitly set before WP is loaded will inherit the default value for that argument.
 * It is therefore possible for a child theme to pass minimal arguments as long as the parent theme specifies others that may be necessary.
 * Allows for a child theme to alter callbacks for <head> and preview (previously, calling add_custom_image_header more than once broke things).
 * The just-in-time bits ensure that arguments fall back to default values, that the values of all constants are considered (such as one defined after an old add_custom_image_header call), and that all constants are defined (so as to be backwards compatible).

get_theme_support(): Introduce new second argument, which headers and backgrounds leverage to return an argument. current_theme_supports() already supported checking the truthiness of the argument.
 * For example, get_theme_support( 'custom-header', 'width' ) will return the width specified during registration.
 * If you had wanted the default image, use get_theme_support( 'custom-header', 'default-image' ) instead of HEADER_IMAGE. 

Deprecate remove_custom_image_header(), remove_custom_background(). Use remove_theme_support('custom-header'), 'custom-background'.

Deprecate short-lived custom-header-uploads internal support; this is now (bool) 'uploads' for add_theme_support().

New 3.4 functions renamed or removed: Rename get_current_header_data() to get_custom_header(). Remove get_header_image_width() and _height() in favor of get_custom_header()->width and height.



git-svn-id: http://core.svn.wordpress.org/trunk@20212 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix copy-paste issue. see #20249.

git-svn-id: http://core.svn.wordpress.org/trunk@20213 1a063a9b-81f0-0310-95a4-ce76da25c4cd