Webclient: open menus with ENTER or SPACE (#534).

When a menu button has focus its menu can be opened by pressing ENTER or SPACE.
Thanks, Malte, for reporting.

One issue remain: because in a just opened menu no item is focused pressing ENTER/SPACE (which normally selects the focused item) in this situation throws a client-side error.

Extend docu for GUIToolkit Button.

See #534.