Add support for dynamic properties for QObjects in JavaScript

In QtScript you could add properties to a JS object that wraps a QObject.
Depending on the wrap option the property was either stored on the JavaScript
side or as dynamic QObject property.

In QJSEngine/QJSValue, neither was supported - properties could not be added.
For QObjects wrapped in JavaScript that weren't created by QML, we can restore
the behavior of storing dynamically added properties as JavaScript properties.

This makes porting from QtScript to QJS* much easier.

Task-number: QTBUG-37408
Change-Id: I5ef1f379c08c3d84de9bdcac9b6a9397238064de
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Fix unreliable behavior of array methods on qml list properties

Array methods such as forEach rely on the hasProperty boolean of getIndexed to
be set appropriately. Some getIndexed implementation - such as the
QQmlListProperty one - didn't initialize it correctly and therefore the
behavior was undefined.

Task-number: QTBUG-38088
Change-Id: I34bc3136d8cc2bc280397d0c4d5051e7d72269e8
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reduce memory pressure on JS stack when garbage collecting

As the identifier table grows with long running programs, we may end up
allocating more identifiers than we have space left on the JS stack for them
alongside all the other objects in the environment. To mitigate this, we can
simply treat the identifiers as roots, mark them manually and only end up
putting sub-strings onto the JS stack if necessary.

Task-number: QTBUG-36183
Change-Id: Ie6994555305c84b007860792d066a8df60089847
Reviewed-by: Lars Knoll <lars.knoll@digia.com>