Deprecate QScriptEngine::{push,pop}Context()

See http://doc.trolltech.com/4.7/qscriptengine.html#pushContext
pushContext() is used to open up a new "evaluation scope"; effectively, it creates a new JS stack frame in which JavaScript can be evaluated without "polluting" the outer scope.
This is useful functionality that we don't want to drop.
However, push/popContext() is too low-level; we would like not to expose the stack frames directly.
An alternative API could be something like this:

QScriptEngine eng;
{
    QScriptEvaluationScope scope(&eng);
    eng.evaluate("var foo = 123"); // "foo" would be defined in local scope
}
eng.evaluate("foo"); // ReferenceError - the scope where "foo" was defined no longer exists