Evaluate whether to use JS syntax and V8 for QRegExp

The regexp engine in QtCore is getting rather old. V8 provides a much more performant engine that we could use. With JS becoming more important in Qt we need to evaluate whether we should support JS type regexps and whether this should be the default syntax in Qt 5.

• Evaluate possibility to implement QRegExp with V8
• Evaluate risk of changing default syntax to JS

A silent breakage of the syntax looks too risky. One possible path forward could be:

• Move QRegExp into it's own add-on module.
• Add a QRegularExpression in QtCore that uses V8 and the JS syntax
• Change all API in Qt to use QRegularExpression instead of QRegExp
• Provide a cast operator from QRegExp to QRegularExpression for source compatibility'