This patch performs the following cleanup on JSInterface_IGUIObject:
* Replace `JS_THIS_OBJECT` deprecated in SpiderMonkey 61 with JS::CallArgs or JS::CallReceiver thisv(), refs D1699, https://bugzilla.mozilla.org/show_bug.cgi?id=1255800
* Unify the 7 copies of the `JS_GetInstancePrivate` call and its dependencies.
* Unify the 2 copies of the GetProperty and SetProperty attribute handling code.
* Unify the 2 copies of the `GetEventName` code.
* Move GetParent, GetChildren, GetName, GetEventHandler, SetEventHandler to a custom function.
* Move a conversion to ToJSVal<CRect>.
* Rename e to obj.
This makes JSI_IGUIObject::getProperty and setProperty agnostic of the handling per property,
thus enabled developers to change the way the properties and functions are accessed and set.