Move WP_Editor to a _WP_Editors encapsulation.

 * WP_Editor will return in 3.4 as a one true API for editor instances. Stick to wp_editor() for now.
 * TinyMCE can now be forced on with tinymce = true. It defaults to the value for user_can_richedit().
 * Restores wp_default_editor(), wp_link_query(), wp_link_dialog(), wp_fullscreen_html().
fixes #19320.


git-svn-id: https://core.svn.wordpress.org/trunk@19420 1a063a9b-81f0-0310-95a4-ce76da25c4cd