Improve the highlighting in the SolrSearch page and allow up to three highlighted content regions

Currently we have only one highlighted region for each result. It would be more meaningful for the user if he would see at least 3 such regions/occurrences inside the result to better asses the relevance of the result. (a bit similar to what Google displays under each result).

This work will also fix the current non-functioning state of the highlighting feature in the SolrSearch page due to the recent multilingual changes introduced by XWIKI-8579.