Menu Customizer: remove check for existence of WP_Customize_Manager::add_panel() - it's been in core for over a month and 4.0 is required for this plugin.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@967926 b8457f37-d9ea-0310-8a92-e5e31aec5664
Menu Customizer: update theme location section description to refer to the Customizer versions of both widgets and menus.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@967927 b8457f37-d9ea-0310-8a92-e5e31aec5664