Smart menus are not scrollable (vertical scrolling in long menu list)

[wiebkemueller-hsh]

This is a follow-up to #300

As a user, I want to scroll through the smart menu if it is cutoff vertically, so that I am still able to access every single entry. Currently, scrolling (in either direction) is not possible which leaves me as a user unable to reach said entries in the smart menu. As this is a standard operation throughout the web, the menu is not intuitive for me as a user.

Ideally I should be able to scroll independently from any circumstances: the scrolling operation should not depend on either the amount of list entries nor the available viewport. First and foremost this should be fixed for vertical cut offs, but fixing scrolling in a horizontal plane would also alleviate #356. As I am atleast able to read the entries then.

[wiebkemueller-hsh]

Specifying this bug, instructions to reproduce:

1. Create a smart menu
2. create entries in the first level with no submenus but inline entries, dynamic.
3. in order to reproduce that the list is not scrollable there have to be enough entries in the list

[wiebkemueller-hsh]

preferably with visible scroll bar.