Toolbar: Use `add_node()` instead of `add_menu()` in core.

This patch replaces all references to the add_menu() method with the add_node() one. (Also some code structure modifications for wp_admin_bar_appearance_menu().)

Fixes: #19647
Props: linuxologos, paulschreiber, morganestes, akibjorklund, nacin, whyisjake.

Built from https://develop.svn.wordpress.org/trunk@46642


git-svn-id: https://core.svn.wordpress.org/trunk@46442 1a063a9b-81f0-0310-95a4-ce76da25c4cd