[BUGFIX] Workspace selector in top toolbar cannot scroll

If there are too many workspaces defined, some of them cannot
be accessed using the workspace selector in in the top toolbar
due to a missing scrolling behaviour of that component.

Fixes: #11014
Releases: 6.2, 6.1, 6.0
Change-Id: Iaae51ebb2524116b8673a9cc81d612fd845b2d29
Reviewed-on: https://review.typo3.org/21912
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind