Modify the class AcSearchIndex

This commit contains changes to the AcSearchIndex that make it
use some of the new Contao 3.2 features.

Signed-off-by: Leo Unglaub <leo.unglaub@2create.at>
Modify the frontend module and use some new Contao 3 stuff

Signed-off-by: Leo Unglaub <leo.unglaub@2create.at>