Missing custom CSS classes for backend navigation items

[qzminski]

Since Contao 4.5 the backend menu items lack the custom CSS classes functionality. It was present in 4.4 but is no longer in the new Twig template implementation.

Contao 4.4: https://github.com/contao/core-bundle/blob/4.4/src/Resources/contao/templates/backend/be_main.html5#L64
Contao 4.5: https://github.com/contao/core-bundle/blob/4.5/src/Resources/views/Backend/be_menu.html.twig#L7

Some extensions rely on this, e.g. easy_themes that is adding a custom CSS class via getUserNavigation hook:
https://github.com/terminal42/contao-easy_themes/blob/master/EasyThemes.php#L278

Issue originated from terminal42/contao-easy_themes#38

[Toflar]

/cc @sheeep

[leofeyer]

Fixed in b5fc53d.