MDL-43738 behat: More info about how to get/set fields
MDL-43738 behat: Refactoring the field type guesser

The previous behaviour delegates the responsability
of overwriting the 3 public methods to the child
classes, now the field type or moodleform field is
guessed earlier in the execution flow.

Other changes introduced here:
- Fix wrong moodleform detection when there is a form
  in the page but the field we are dealing with is
  not inside it.