MDL-43738 behat: Normalization and major refactoring of getters and setters

Every single step that sets or gets a value from a field
has been updated to follow the same behaviour both when
using it through a single step or through generic steps
like "I fill the moodle form with:", to resume all the
changes:
- Created a behat_form_group to re-guess the field type
  and act appropriately setting and getting it's value
- Normalize all getters and setters to use behat_form_field
  children
- Complete behat_form_checkbox to trigger the appropiate JS
  event needed to perform some JS scripts that are listening
- Refactor MDL-43713 multi-select changes and remove
  the two new steps introduced there as that behaviour can
  be managed from the generic getter
MDL-43738 behat: New step to adapt to radio buttons behaviour

Now we interact with radio fields through it's value
or text.
MDL-43738 behat: Backporting new 2.7 methods to 2.6

We encourage backporting new feature files, adding
here also these Moodle 2.7 new step definitions so
future features will be easier to backport as we
would be able to directly cherry-pick them.