Render fields into menu_link's #below, rather than replacing menu_link altogether

Currently, menu_fields replaces the 'menu_link' theme function altogether. This is kind of extreme and will undo any styling that the theme was doing to the top-level <li ...></li> tag and the primary link.

For example, in a Bootstrap-based theme, it'll add a number of classes and data-* attributes to the <li ...></li> tag to turn the menu link into a dropdown, and add a 'caret' icon to the text of the primary link.

Because menu_fields replaces the 'menu_link' theme function completely, all these customizations are lost, AND the only way to add them back is to alter the theme registry, removing menu_fields_link() and redoing most of the code in that function. :-/

Proposal

Instead of replacing the 'menu_link' theme function entirely, use hook_preprocess_menu_link() to render the menu fields and use them to replace $vars['element']['#below']. This way the theme is still reponsible for rendering the menu link its normal way.

This has a couple of cool side benefits:

• In Bootstrap-based themes, it's probably already rendering the children of the main menu as a dropdown, so menu_fields will just work!
• This fixes #2460187: menu_fields_theme_registry_menu_link will be wrong if there are multiple themes because we no longer need to try and store the original 'menu_link' theme function

I'll post a proof-of-concept patch soon-ish...