Container: add support for URLs and Components

Container (inherited by SwipeView/TabBar) provides API for adding and inserting items dynamically:

• void addItem(Item item)
• void insertItem(int index, Item item)
  Given that the purpose of this API is dynamic addition and removal, it typically also involves dynamic creation of items. Therefore Container should make it more convenient by accepting URLs and Components in addition to plain Items and do the job of creating an instance and initializing the properties, just like StackView does:
• Item addItem(var item, jsobject properties)
• Item insertItem(int index, var item, jsobject properties)