Demo: getSecure TeX see MathSearch

* Keep the tex output of texvc and treat png and tex file in the same way
* Fix: emtpty tex string on load from database
* new Method: MathTexvc::getSecuredTex
* Fixed: Broken tests

Change-Id: Iaaf5c7ae018eeb0d014bbc2e06c7ffa7f40e14dc
Merge branch 'LaTeXML' of ssh://gerrit.wikimedia.org:29418/mediawiki/extensions/Math into LaTeXML