"New Window" command has to wait for initial "about:blank" being loaded

[whimboo]

The New Window command doesn't actually await the load of the initial about:blank page within the newly opened tab or window. Given that the load of about:blank is not synchronous anymore in most of the browsers, it's also kinda complex.

To prevent an early return from the command affecting the following command(s) by an unexpected navigation (issue #1308) it has to wait until the page load has been reached a state as specified in the page load strategies.

CC'ing @jgraham, @shs96c, and @AutomatedTester.