Improve "Navigate by" layout

Implemented some suggestions from https://freedomsponsors.org/issue/391/navigation-button-icon-does-not-scale-and-gets-cut-off#comment-917
* Place the up/down arrows on the right of the button
* Use up/down arrows instead of right/left
* Added a separator between the navigation buttons and the search label

Change-Id: I509756db2fc0c1f60a6b1d6f14e6ed9299d7e597
Reviewed-on: https://gerrit.libreoffice.org/10613
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
(cherry picked from commit 21bd05d61febe5e99df790fdad572ec731a15213)
Reviewed-on: https://gerrit.libreoffice.org/10652