com_search Highlighter SPANs wrong position with german umlauts ans corrupted if overlapping searchwords 

#### Issue (copy from old JC)
Problems
1. Create an article with some german umlauts (ü, ä etc.). - Search in FE with com_search for a word near but after a word with umlaut. - The highlight-SPAN starts at a wrong position. It's moved 1 position to the right for each umlaut before the search hit.
2. Create an article with some phrases and e.g. word "Potsdam". - Search for "Potsdam dam". - 2 highlighter SPANs will be set but corrupt because of overlapping search words.

### Old JC
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33492

### PDF by @illovo that explain the patch
http://joomlacode.org/gf/download/trackeritem/33492/158347/Joomla-3.2.3-corrupted-highlighter-SPANs.pdf

### Tests
It is tested on a german forum here (by gbuechner):
http://www.joomlaportal.de/joomla-3-x-allgemeine-fragen/311023-suchergebnisse-merkw-rdige-textmarkierung.html
and here (by k78)
http://www.joomlaportal.de/joomla-3-x-allgemeine-fragen/313194-suchbegriffe-suchresultaten-werden-falsch-markiert-protostar-template.html

### Based on
This based on a closed but still valid PR (https://github.com/joomla/joomla-cms/pull/3337) by @illovo so the kudos goes to @illovo
I only do some CS work on it. So thanks @illovo