Make menu_tree() more theme-friendly

When trying to output a highly customized menu tree, the existing menu_tree() is not very theme-friendly. The convention always employed is to override calls to menu_tree() to a custom one and its children. Just notice that admin_menu is doing the same.

e.g.

<?php
function _my_menu_tree($menu_name = 'navigation') {
...
  $menu_output[$menu_name] = _my_menu_tree_output($tree);
...
}

function _my_menu_tree_output($tree) {
...
  $link = _my_top_menu_item_link($data['link']);  // theme for 1st level menu
...
  $output .=  theme('menu_item', ..., _my_submenu_tree_output($data['below']), ...);
...
}

function _my_submenu_tree_output($tree) {
...
  $link = _my_sub_menu_item_link($data['link']);  // theme for sublevel menu
...
  $output .=  theme('menu_item', ..., _my_submenu_tree_output($data['below']), ...);
...
}
?>

which in turn need to duplicate the menu_tree_output() and often menu_item_link() as a custom function.

Another shortcoming is they are lacking the knowledge of the menu hierarchy (currently only come across menu_item_link may need that info). Often during themeing, we'll theme main menu differently especially a simple drop down menu, not to mention other more complex menu display (showing menu with inline description, themeing individual submenu differently, multiple columns etc).

It gets complicated during version upgrade as those codes need to be recoded if there are changes and could pose a security leak. A more theme-friendly menu_tree is needed in order to make drupal looks nicer. =)