[QtScript/JavaScriptCore] Allow setting timeout check interval, and make timeout check virtual

Patch: http://gitorious.org/+qtwebkit-developers/qtwebkit/qtwebkit-qtscript-integration/commit/1710893a8320b0abdc615fe9f52d10395bfd36cf

Reason: We want a timeout hook, so we can do e.g. event processing (for QScriptEngine::setProcessEventsInterval()).

This patch can't be upstreamed as-is. It needs a justification / real usage upstream.

Proposed action: Try to introduce a timeout/interval notification in the JSC C API, possibly using this patch to provide the plumbing.

Dropping this patch would cause setProcessEventsInterval() and abortEvaluation() to stop working.
An alternative might be to install a debugger client (so we get callbacks at every statement and can measure the time ourselves), at the potential expense of greatly reduced performance.