Bug 20155: Improve readability of OPAC header language menu

Bug 20155: Improve readability of OPAC header language menu

This patch makes some minor HTML and CSS changes in order to make the
header's language-chooser menu more readable and consistent with the
Lists menu.

To test, apply this patch and the patch with the compiled CSS.

- Install and enable more than one translation.
- Set the OpacLangSelectorMode to either "top" or "both top and footer."
- View the OPAC's header menu and confirm that the links and menus look
  correct whether or not a user is logged in.

Signed-off-by: Jon Knight <J.P.Knight@lboro.ac.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>