Use one script engine and one script debugger to avoid slow down of the skin parsing 

+ Using the debugger is much faster than before
+ The debugger is disabled on the spinny widget to avoid segfault (multithread?).
+ The global object is preserved from a skin context to it's children (as when there is one scriptEngine per context).
+ The translation js api is enabled (http://qt-project.org/doc/qt-4.8/qscriptengine.html#installTranslatorFunctions)
+ Removal of a circular reference of the global object in the svg extension 

I have a really fast SSD here so I didn't realize the slow down was so massive. Please tell me if there is another bottleneck on different setups.
It was simply the result of grep -nHIirE -- skincontext[^=]*= in the mixxx's folder. The two other lines in the result are useless : 
```
./src/skin/legacyskinparser.cpp:1346:    SkinContext* pOldContext = m_pContext;
./src/widget/weffectparameterbase.h:18:    virtual void setup(QDomNode node, const SkinContext& context) = 0;
./src/waveform/renderers/waveformrendererabstract.h:18:    virtual void setup(const QDomNode& node, const SkinContext& context) = 0;
```

I already removed it from my branch. Should I cancel it?