Upstream QtScript API to WebKit

This task collects all tasks related to upstreaming the QtScript API to WebKit.
This work is tracked upstream at https://bugs.webkit.org/show_bug.cgi?id=31863

The implementation of the QtScript API in Qt as of 4.6 relies on JavaScriptCore internals. It should migrate to only using the JSC C API, since that's what's supported in the WebKit project. Using internals 1) causes maintenance burden and risk because internals can change at any time, and 2) precludes the possibility of using JSC in shared library form, since not all symbols are exported.

In many cases there is a 1-to-1 mapping between the QtScript API and JSC C API, but where that's not the case we need to figure out if/how the C API can be extended.

Unit tests and microbenchmarks should be added incrementally as each piece of functionality is upstreamed.