Behat 3 upgrade

Upgrade to Behat 3

Changes:
- [x] Minor update on sentences
- [X] Update DemoContext to reflect changes of Behat Upgrade
- [x] Update ContentContext to reflect changes of Behat Upgrade

To do:
- [x] pass on that PR (https://github.com/ezsystems/ezpublish-community/pull/182)
Why do we mix "to click on" and "to click at" ? Not to mention that "to click at" is usually restricted to accurate positions, such as "click at X, Y". Could we make this consistent ?