Added system settings to enable customization of the top bar navigation (main/user)

### What does it do ?

Adds 2 settings to allow building custom "main menu"

### Why is it needed ?

Considering 2.3 introduced some "containers" to pull top menu items from, this allows developers to set which containers to make use of (using `main_nav_parent` & `user_nav_parent` settings).

This brings the ability to build different top menus, which could be defined in user/group settings. That way, we can tailor custom UX according to the kind of user using the manager (content editor, developers...)

### To do before merge

- [x] create appropriate lexicon strings


Edit : another use case could be to handle custom built "main navigation" for some pages, ie. the following plugin : 

```
<?php
/**
 * @var modX $modx
 * @var array $scriptProperties
 *
 * @event OnManagerPageBeforeInit
 */

if (isset($_GET['a']) && $_GET['a'] === 'system/event') {
    $modx->setOption('main_nav_parent', 'container_for_logs');
}

return;
```

Next interesting step regarding modMenu would be to prevent `modMenu.text` being unique (see #10375)