[TASK] Improve namings of CheckboxElement

Use better namings for variables in CheckboxElement.

Count of selectedItems is done multiple times.
Better is to do the counting once and use the result in the
other places.

Resolves: #62012
Releases: master
Change-Id: Id0cced22902dafc33ef086a964e3f9a61891bff6
Reviewed-on: http://review.typo3.org/33122
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>