[REG] QtScript is not fully reentrant

With the switch to using JavaScriptCore in 4.6, QtScript is no longer reentrant. This is because JavaScriptCore is not reentrant, and QtScript does not take care to lock resources in relevant places. This can cause e.g. the attached application (main.cpp) to crash.

Either JavaScriptCore should be made reentrant, or QtScript should do the necessary locking.
There is a feature in JSC, ENABLE_MULTIPLE_THREADS, that's also relevant to this issue. QtScript currently doesn't enable this feature because it's not supported on all platforms.