Updates to WP_Theme, wp_get_themes(), and related deprecated functions, after...

Updates to WP_Theme, wp_get_themes(), and related deprecated functions, after [UT570] [UT578] [UT579]. see #20103.
 * ['Template Files'] and ['Stylesheet Files'] need to return files from the parent theme as well.
 * Don't strip links from the Author header. Some themes rely on the previous behavior, such as to link multiple authors (Sandbox, for example.) Don't restore links to the Name, that's just a bad idea.
 * Ensure we are always passing around arrays in get_files/scandir.
 * Better inline doc for wp_get_themes() arguments.
 * Introduce a 'force' flag for search_theme_directories() to re-scan, rather than return the cache. We will use this to re-build the theme_roots transient in get_theme_roots(), but it is more helpful for unit tests. Since search_theme_directories() is cached, don't cache again in wp_get_themes(). (Again benefits testing.)
 * Handle duplicate theme names in the old get_themes() when two themes match (and neither are a default theme, which is already handled). wp_get_themes() will consider both names to be the same; this is just for back compat since get_themes() is keyed by name.
 * Include an old array key in wp_broken_themes().



git-svn-id: https://core.svn.wordpress.org/trunk@20193 1a063a9b-81f0-0310-95a4-ce76da25c4cd