Handle native modals for alert, confirm and prompt

[adeprez]

When a website calls window.alert, window.confirm and window.prompt it opens a browser popup modal. It will block navigation and any other possible interaction. Therefore, we'd like to make the World Model aware of these modal windows.

The navigation engine should be updated to describe open modals and to interact with them. A solution would be to return the modal as a fake HTML dialog with its actions - and optionaly its input field as HTML.

e.g for

confirm("Please input");

Retrieved HTML would be:

<dialog>
  Please input
  <input xpath="/dialog/input" />
  <button xpath="/dialog/button">Cancel</button>
  <button xpath="/dialog/button[2]">OK</button>
</dialog>