* Getters and Helpers: Introduces a series of methods to allow for easy generation of headers for display, and other theme metadata, including page templates.
* Screenshots: Handles support for multiple screenshots. (see # Additional screenshots must be PNG and start with screenshot-2.png, and be sequential to be counted. see #19816.
* Error Handling: Broken themes have a WP_Error object attached to them.
* Caching: Introduces a wp_cache_themes_persistently filter (also in [20020]) to enable persistent caching of all filesystem and sanitization operations normally handled by WP_Theme (and formerly get_file_data() and get_themes()). Themes are cached individually and across five different cache keys for different data pieces.
* Compatibility: A WP_Theme object is backwards compatible with a theme's array formerly returned by get_themes() and get_theme(), and an stdClass object formerly returned by current_theme_info().
* i18n/L10n: Theme headers are now localizable with proper Text Domain and Domain Path headers, like plugins. (Language packs may remove the requirement for headers.) For page templates, see #6007 (not fixed yet, but will be easy now). For headers, fixes#15858.
* PHP and CSS files: New methods that fetch a list of theme files (for the theme editor) only on demand, rather than only loading them into memory. fixes#11214.
Functions deprecated:
* get_themes(), get_allowed_themes() and get_broken_themes() -- use wp_get_themes()
* get_theme() and current_theme_info() -- use wp_get_theme()
* get_site_allowed_themes() -- use WP_Theme::get_allowed_on_network()
* wpmu_get_blog_allowedthemes() -- use WP_theme::get_allowed_on_site()
see also [20016], [20018], [20019], [20020], [20021], [20022], [20025], [20026], [20027]. also fixes#19244.
see #20103.
git-svn-id: http://svn.automattic.com/wordpress/trunk@20029 1a063a9b-81f0-0310-95a4-ce76da25c4cd
