Add the ability to validate a step before going to the next one

For example, in the "Creating a page" tour, we need to validate:

• if the user actually entered a "Page Title", see create_input.png
• if the page doesn't contain errors and the user can go to the next step, see create_button.png (for example the page name already existed, etc.)

Withouth validation we are just praying that the user doesn't stray from the scenario, otherwise he will lose the tour.