MDL-43713 behat: improve multi-select support

This patch implements:

1) Normalization of options. Before the patch options
in a select were being returned as "op1 op2 op3" by selenium
and "op1 op2 op3" by goutte. With the patch, those lists
are always returned like "op1, op2, op3". If real commas are
needed when handling multiple selects they should
be escaped with backslash in feature files.

2) Support for selecting multiple options. Before the patch
only one option was selected and a new selection was cleaning the
previous one. With the patch it's possible to pass "op1, op2" in
these steps:
  - I fill the moodle form with (table)
  - I select "OPTION_STRING" from "SELECT_STRING"

3) Ability to match multiple options in this steps. Before the
patch matching of multiple was really random, now every every
passed option ("opt1, opt2") is individually verified. It applies
to these 2 steps:
  - the "ELEMENT" select box should contain "OPTIONS"
  - the "ELEMENT" select box should not contain "OPTIONS"

4) Two new steps able to verify if a form have some options selected or no:
  - the "ELEMENT" select box should contain "OPTIONS" selected
  - the "ELEMENT" select box should contain "OPTIONS" not selected

5) Change get_value from xpath search to Mink's getValue() that is immediate
(does not need form submission) and works for all browsers but Safari, that
fails because of the extra ->click() issued.

Note all the changes 1-4 only affect to multi-select fields. Single
selects should continue working 100% the same.

The change 5) causes Safari to fail. The problem has been traced down to
the extra ->click() present there. Anyway there are not test cases
requiring that "immediate" evaluation right now. Only the special feature
file attached verifies it.
MDL-43738 behat: Functional tests for fields management

We are cheating with this .feature file as this
is not a behaviour and we are just testing the
framework, but in this case I think that everybody
will agree on the need of doing it, as every browser
behaves differently and minor changes in the getters
and setters can easily end up breaking the behaviours.

All credit goes to Eloy Lafuente (stronk7).
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.