Вопрос: Как в wordpress зарегистрировать и добавить выпадающее при наведении меню?

Лучший ответ: скидываю свою рабочую заготовку:
functions.phpregister_nav_menus(array(
	'top' => 'Верхнее',
	'bottom' => 'Внизу'
));
if (!class_exists('bootstrap_menu')) {
	class bootstrap_menu extends Walker_Nav_Menu {
		private $open_submenu_on_hover;
		function __construct($open_submenu_on_hover = true) {
	        $this->open_submenu_on_hover = $open_submenu_on_hover;
	    }
		function start_lvl(&$output, $depth = 0, $args = array()) {
			$output .= "\n<ul class=\"dropdown-menu\">\n";
		}
		function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
			$item_html = '';
			parent::start_el($item_html, $item, $depth, $args);
			if ( $item->is_dropdown && $depth === 0 ) {
			   if (!$this->open_submenu_on_hover) $item_html = str_replace('<a', '<a class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false"', $item_html);
			   $item_html = str_replace('</a>', ' <b class="caret"></b></a>', $item_html);
			}
			$output .= $item_html;
		}
		function display_element($element, &$children_elements, $max_depth, $depth = 0, $args, &$output) {
			if ( $element->current ) $element->classes[] = 'active';
			$element->is_dropdown = !empty( $children_elements[$element->ID] );
			if ( $element->is_dropdown ) {
			    if ( $depth === 0 ) {
			        $element->classes[] = 'dropdown';
			        if ($this->open_submenu_on_hover) $element->classes[] = 'show-on-hover';
			    } elseif ( $depth === 1 ) {
			        $element->classes[] = 'dropdown-submenu';
			    }
			}
			parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);
		}
	}
}
header.php<nav class="navbar navbar-default">
						<div class="navbar-header">
							<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#topnav" aria-expanded="false">
								<span class="sr-only">Меню</span>
								<span class="icon-bar"></span>
								<span class="icon-bar"></span>
								<span class="icon-bar"></span>
							</button>
						</div>
						<div class="collapse navbar-collapse" id="topnav">
							<?php $args = array( 
								'theme_location' => 'top',
								'container'=> false,
						  		'menu_id' => 'top-nav-ul',
						  		'items_wrap' => '<ul id="%1$s" class="nav navbar-nav %2$s">%3$s</ul>',
								'menu_class' => 'top-menu',
						  		'walker' => new bootstrap_menu(true)		  		
					  			);
								wp_nav_menu($args);
							?>
						</div>
					</nav>@media (min-width: 979px) { 
	ul.nav li.dropdown.show-on-hover:hover > ul.dropdown-menu {
		display: block;
	}
	.comment-list .children {
		padding: 0;
		margin-left: 74px;
		padding-bottom: 15px;
	}
	.comment-list .media {
		margin-top: 0;
	}
}
Взято из: https://github.com/saxap/clean-wp-template
Удачи в разработках =)

Худший ответ: скидываю свою рабочую заготовку:
functions.phpregister_nav_menus(array(
	'top' => 'Верхнее',
	'bottom' => 'Внизу'
));
if (!class_exists('bootstrap_menu')) {
	class bootstrap_menu extends Walker_Nav_Menu {
		private $open_submenu_on_hover;
		function __construct($open_submenu_on_hover = true) {
	        $this->open_submenu_on_hover = $open_submenu_on_hover;
	    }
		function start_lvl(&$output, $depth = 0, $args = array()) {
			$output .= "\n<ul class=\"dropdown-menu\">\n";
		}
		function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
			$item_html = '';
			parent::start_el($item_html, $item, $depth, $args);
			if ( $item->is_dropdown && $depth === 0 ) {
			   if (!$this->open_submenu_on_hover) $item_html = str_replace('<a', '<a class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false"', $item_html);
			   $item_html = str_replace('</a>', ' <b class="caret"></b></a>', $item_html);
			}
			$output .= $item_html;
		}
		function display_element($element, &$children_elements, $max_depth, $depth = 0, $args, &$output) {
			if ( $element->current ) $element->classes[] = 'active';
			$element->is_dropdown = !empty( $children_elements[$element->ID] );
			if ( $element->is_dropdown ) {
			    if ( $depth === 0 ) {
			        $element->classes[] = 'dropdown';
			        if ($this->open_submenu_on_hover) $element->classes[] = 'show-on-hover';
			    } elseif ( $depth === 1 ) {
			        $element->classes[] = 'dropdown-submenu';
			    }
			}
			parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);
		}
	}
}
header.php<nav class="navbar navbar-default">
						<div class="navbar-header">
							<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#topnav" aria-expanded="false">
								<span class="sr-only">Меню</span>
								<span class="icon-bar"></span>
								<span class="icon-bar"></span>
								<span class="icon-bar"></span>
							</button>
						</div>
						<div class="collapse navbar-collapse" id="topnav">
							<?php $args = array( 
								'theme_location' => 'top',
								'container'=> false,
						  		'menu_id' => 'top-nav-ul',
						  		'items_wrap' => '<ul id="%1$s" class="nav navbar-nav %2$s">%3$s</ul>',
								'menu_class' => 'top-menu',
						  		'walker' => new bootstrap_menu(true)		  		
					  			);
								wp_nav_menu($args);
							?>
						</div>
					</nav>@media (min-width: 979px) { 
	ul.nav li.dropdown.show-on-hover:hover > ul.dropdown-menu {
		display: block;
	}
	.comment-list .children {
		padding: 0;
		margin-left: 74px;
		padding-bottom: 15px;
	}
	.comment-list .media {
		margin-top: 0;
	}
}
Взято из: https://github.com/saxap/clean-wp-template
Удачи в разработках =)