functions.php

<?php

function orbis_timesheets_admin_menu() {
	add_menu_page(
		__( 'Orbis Timesheets', 'orbis_timesheets' ),
		__( 'Timesheets', 'orbis_timesheets' ),
		'manage_options',
		'orbis_timesheets',
		'__return_false',
		'dashicons-clock',
		40
	);

	add_submenu_page(
		'orbis_timesheets', // parent_slug
		__( 'Orbis Timesheets Settings', 'orbis_timesheets' ), // page_title
		__( 'Settings', 'orbis_timesheets' ), // menu_title
		'manage_options', // capability
		'orbis_timesheets_settings', // menu_slug
		'__return_false' // function
	);

	global $submenu;

	/*
	 * Add taxonomy 'orbis_timesheets_activity' to sub menu
	 * @see https://github.com/WordPress/WordPress/blob/3.9.1/wp-admin/menu.php#L59
	 */
	if ( isset( $submenu['orbis_timesheets'] ) ) {
		$tax = get_taxonomy( 'orbis_timesheets_activity' );

		$submenu['orbis_timesheets'][] = array(
			esc_attr( $tax->labels->menu_name ),
			$tax->cap->manage_terms,
			add_query_arg( 'taxonomy', $tax->name, 'edit-tags.php' )
		);
	}
}

add_action( 'admin_menu', 'orbis_timesheets_admin_menu' );