[2.3] - topnav menu

This is an attempt to make the `header.php` controller a little bit more readable, and lower the code duplication (while it could certainly benefit some more abstraction).

It also brings : 

* ability to use "third party" lexicons in main menus
* user navigation generation according to the "usernav" container (so allow some more user customization)
* ability to user markup (using `modMenu.icon`) for first level menu entries
* ability to use nested Smarty tags (so far, only for the usernav)

This is mainly to get reviewed. Will be squashed if approved :smile: