[FEATURE] Make linking of section titles configurable

Adding the TS property
plugin.tx_indexedsearch.linkSectionTitles = boolean
which allows turing on/off linking of the sections' titles.
Default is 1 to keep current behaviour.

Resolves: #16794
Releases: master
Change-Id: I0aa32ed49a16dcdd737907324a69d15057598920
Reviewed-on: http://review.typo3.org/20193
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>