Use an autowired UiElement

[jacquesbh]

• Be sure the UiElement is autowired
• Use a trait for UiElement implementation
• Use ui_element variable instead of uiElement, in twig
• Add Google Maps Element in test application

I've added the Google Maps Ui Element with a custom class as Ui Element.

The setAutowired(true) allows the container to asutowire the Ui Element, which is great and better than before!