[BUGFIX] Invisible icon blocks mousefocus of non-required inputfields

Set z-index for clear.gif, so a backend user can select the text
inside input fields. Now it's also possible to set focus on the
input field by clicking at the beginning of it.

Fixes: #34042
Releases: 6.0, 6.1, 6.2
Change-Id: Icacbaf99d46da979a4eae715f97108df39e0728e
Reviewed-on: https://review.typo3.org/21608
Reviewed-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters