Move the browser window of an actor to the foreground when acting as him

Each time a new actor appears in a scenario the browser window of the
new actor is put in front of the browser windows of the previous actors.
Before, when acting again as a previous actor his browser window stayed
in the background; in most cases everything worked fine even if the
window was in the background, but due to a bug in the Firefox driver of
Selenium and/or maybe in Firefox itself when the window was in the
background it was not possible to set the value of an input field that
had a range selected.

Now, when acting again as a previous actor his browser window is brought
to the foreground. This prevents the bug from manifesting, but also
reflects better how a user would interact with the browser in real life.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>