[BUGFIX] Missing bottom margin in the page tree

This fixes the UI issue with the deletion overlay and drag&drop
with nodes at the very bottom of the page tree viewport too.

Change-Id: I5cee23ea9441fcd1910a998547772bc3f2e53fb5
Fixes: #35182
Releases: 6.0, 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/10411
Reviewed-by: Philipp Gampe
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack