[sailfishbrowser] Setup gecko embedding explicitly. Contributes to JB#5485
[sailfishbrowser] Setup gecko embedding explicitly. Contributes to JB#5485

Previously the factory method QMozContext::GetInstance() not only
created singleton, but also could have side effects like scheduling
gecko embedding setup and shutsown procedures. Now this scheduling
should be done explicitly.

alinelena/sailfish-browser,sailfishos/sailfish-browser,alinelena/sailfish-browser,rojkov/sailfish-browser,alinelena/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,rojkov/sailfish-browser,alinelena/sailfish-browser,rojkov/sailfish-browser,sailfishos/sailfish-browser