Don't leak ScopeChainNode's

Don't leak ScopeChainNode's

Create a sub-scope of the global scope chain, rather than a completely new scope
chain.  Leaks are difficult to autotest, but an autotest for
QScriptDeclarativeClass::pushCleanContext() was added to ensure its behavior doesn't
regress.  To reproduce the leak (prior to this change) use:

while (true) {
    QScriptDeclarativeClass::pushCleanContext(&engine);
    engine.popContext();
}

Change-Id: I41ac61ea1664da569eb329c8276f2a0bb6d2f1f7
Task-number: QTBUG-17166
Reviewed-by: Martin Jones
(cherry picked from commit 3bc6f8d8dd630cd0298e27fc4b7430d2bf73a232)

Conflicts:

	tests/auto/declarative/qdeclarativeecmascript/tst_qdeclarativeecmascript.cpp