NEW: Adding some Behat steps for css selectors and Dialog confirmation

This adds 2 new Behat steps one is to click on a element with a specific css selector (i.e. a cross to remove a option from a listbox) and the other step is a confirm Dialog wrapper for the other new step.

This is related to the following issue I have been investigating

https://github.com/silverstripe/silverstripe-cms/issues/730

The 2 new steps have been added so they can be used in a unit test which test if a alert appears when a admin tries to remove there own admin permissions.
I will be raising a pull request for framework which includes a unit test which will make use of these 2 new steps