Allow accessing selectors handler in Session

In the 1.x versions it was possible to use Session::getSelectorsHandler() method to get selectors handler and change it (e.g. add new selectors to it).

In the 2-architecture-changes branch things have changed considerably and now we have (see https://github.com/Behat/Mink/blob/2-architecture-changes/src/Behat/Mink/Session.php#L46):

(DriverInterface + SelectorsHandler + Manipulator) = ElementFinder
(DriverInterface + ElementFinder) = Session

As you can see the SelectorsHandler is now created deep within ElementFinder and ElementFinder itself is created within Session.

If developer has full control over Session creation then of course he can create ElementFinder as he see fits and in the process obtain reference to SelectorsHandler.

However within Behat developer gets already created session via ->getSession() method that he can't change anymore.

Start with src/Behat/Mink/Session.php and trace how Session creates or receives ElementFinder and its SelectorsHandler. Compare that path with Behat's getSession() entry point; done means the selectors handler can be accessed and modified through the session Behat provides without requiring control over Session construction.