Add new step definitions to restrict the scope of clicks and "contains text" assertions

As Eloy commented in MDL-40392, new steps definitions like this can be added:

• I should see "xxxxx" in the "yyyy" "MOODLE_ELEMENT"
• I click on "xxxxx" "SELECTOR" in the "yyyyy" "MOODLE_ELEMENT"

MOODLE_ELEMENT would be a closed list of variables like block, header or region and "yyyyy" would be a var depending on this MOODLE_ELEMENT var used to specify the block name, the form fieldset heading...

Note that SELECTOR is one of the selectors listed in http://docs.moodle.org/dev/Acceptance_testing#Providing_values_to_steps

Until now only the needed step definitions have been added, but now that contributors will begin to add feature files will be better if they have a bigger set of steps which allows them to skip using CSS selectors and XPath as much as possible.