Check/uncheck checkbox/radio button by clicking on it.

Right now check/uncheck behaviour is emulated by:

1. setting `checked` attribute manually
2. firing `change` event manually

```php
$this->executeJsOnXpath($xpath, '{{ELEMENT}}.checked = true');
$script = "Syn.trigger('change', {}, {{ELEMENT}})";
$this->withSyn()->executeJsOnXpath($xpath, $script);
```

This have several drawbacks:
1. `click` event isn't being triggered (user clicks on checkbox to change it's state)
2. if called several times a row, an emulated `change` event is triggered each time (doesn't happen when user is doing that)

I propose to use `isChecked` and `click` methods (of a driver) to simulate user behavior. I've checked, that when calling `click` method of `WebDriver` it does fire `change` and `click` events correctly.