[BUGFIX] Display prev & next icons in workspace module again

As the workspace module is not working with the new icon API yet,
show the icons by adding them in the CSS file directly.

Change-Id: Ib49d8d8ed33034ed0cfdb59a3f2b022fb0ef8fee
Resolves: #71797
Releases: master
Reviewed-on: https://review.typo3.org/45159


Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Ralf Merz <info@merzilla.de>
Tested-by: Ralf Merz <info@merzilla.de>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>