Add support for convenient conversion from JSStringRef to QString
https://bugs.webkit.org/show_bug.cgi?id=109694

Reviewed by Allan Sandfeld Jensen.

Add JSStringCopyQString helper function that allows for the convenient
extraction of a QString out of a JSStringRef.

* API/JSStringRefQt.cpp: Added.
(JSStringCopyQString):
* API/JSStringRefQt.h: Added.
* API/OpaqueJSString.h:
(OpaqueJSString):
(OpaqueJSString::qString):
(OpaqueJSString::OpaqueJSString):
* Target.pri:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145700 268f45cc-cd09-0410-ab3c-d52691b4dbfc