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.
mmm
```
c5d3c05f (RJ Ryan           2014-01-01 21:49:25 -0500  60) SkinContext& SkinContext::operator=(const SkinContext& other) {
c5d3c05f (RJ Ryan           2014-01-01 21:49:25 -0500  61)     m_variables = other.variables();
```
It was probably a really happy new year! ;)

https://github.com/mixxxdj/mixxx/blob/master/src/skin/legacyskinparser.cpp#L1346