feat(pat-validation): Support definition of minimum or maximum number of selections.

[thet]

Hi @cornae

we had the need to validate if at least one checkbox in a group of checkboxes was checked.
This was not yet supported by pat-validation nor by the HTML specficiation.

As a side note - required radio buttons are supported. A required attribute on one or all radio inputs in a group of radio inputs is supported by the HTML specification and validated by pat-validation.

We were in need to quickly support that in one of our projects and I did a implementation which we deployed but still can change if there is anything to improve - behavior wise, parameter naming or whatever.

These parameters were added:

Property	Description	Default	Type
message-min-values	The error message when the minimim number of checked, selected or filled-out fields has not been reached.	You need to select at least %{count} item(s).	String
message-max-values	The error message when the maximum number of checked, selected or filled-out fields has not been reached.	You need to select at most %{count} item(s).	String
min-values	Minimum number of checked, selected or filled out form elements.	null	Integer (or null)
max-values	Maximum number of checked, selected or filled out form elements.	null	Integer (or null)

Note: this does not only work with checkboxes but with any kind of input.
I recognized that we can use the same name for any kind of input - any input, select or textarea. All of the values will be submitted to the server. Therefore this implementation is not only limited to checkboxes but any kind of input.

Along the way I did some other improvements:

• The documentation is updated and extended.

• pat-validation now fully supports input elements outside the form itself, e.g.:

<input name="outside" form="myform" required>
<form id="myform">
</form>
<button form="myform">submit</button>

• pat-validation also fully supports dynamic forms without re-initalizing or sending a special "pat-update" event. You can add form elements even without pat-inject and it should immediately take part in validation.

• And a important one:
  For multiple input elements with the same name the error message was shown wether immediately after the last input which the user interacted with or the the end of a fieldset.checkbox element.
  I have changed that, so that the error message is just shown after the last input with the same name.
  We needed that also for our project and I believe it's better because it works just out of the box and there is no need to add a fieldset.checkbox element.