Webclient: fix long menus (#532, #534).

In long menus, when a (vertical) scrollbar appears:
    - the menu items are not line wrapped (the menu is wide enough).
    - no horizontal scrollbar appears.

=> If one or both is not true for your OS or windowing system please send me an unscaled screenshot of the menu.

Note: you can use the mouse wheel or whatever scrolling gesture to scroll through long menus.
The former upper/lower "scrolling areas" are gone.

See #532.
See #534.