Make WebDriver::getLocator (and WebDriver::getStrictLocator) public

#### What are you trying to achieve?
Write a custom action that uses the WebDriver locator service to locate an element by its selector string.
I would like to do this by getting the web driver module with `$this->getModule('WebDriver')` rather than extending `WebDriver` to keep my testing architecture consistent with other modules I've written.

#### What do you get instead?
Unable to use the functions because they're protected.

```php
/**
* @param $selector
* @return WebDriverBy
* @throws \InvalidArgumentException
*/
protected function getLocator($selector)
{
if ($selector instanceof WebDriverBy) {
return $selector;
}
if (is_array($selector)) {
return $this->getStrictLocator($selector);
}
if (Locator::isID($selector)) {
return WebDriverBy::id(substr($selector, 1));
}
if (Locator::isCSS($selector)) {
return WebDriverBy::cssSelector($selector);
}
if (Locator::isXPath($selector)) {
return WebDriverBy::xpath($selector);
}
throw new \InvalidArgumentException("Only CSS or XPath allowed");
}

// ...and

/**
* @param array $by
* @return WebDriverBy
*/
protected function getStrictLocator(array $by)
{
$type = key($by);
$locator = $by[$type];
switch ($type) {
case 'id':
return WebDriverBy::id($locator);
case 'name':
return WebDriverBy::name($locator);
case 'css':
return WebDriverBy::cssSelector($locator);
case 'xpath':
return WebDriverBy::xpath($locator);
case 'link':
return WebDriverBy::linkText($locator);
case 'class':
return WebDriverBy::className($locator);
default:
throw new MalformedLocatorException(
"$by => $locator",
"Strict locator can be either xpath, css, id, link, class, name: "
);
}
}
```
### Details
It seems to me that these are services that the `WebDriver` module should expose, as some public `WebDriver` functions take `WebDriverBy` arguments (e.g. `WebDriver::findElements`). We need some service to get these `WebDriverBy` arguments, or we won't be able to use the public function calls that are exposed through `$this->getModule('WebDriver');`. Alternatively, maybe the `WebDriver` module should expose some public `findElement` type of function that takes a selector string rather than a `WebDriverBy` instance.

* Codeception version: 2.2.10
* PHP Version: 7.0.10

Apologies if this has already been discussed, I didn't see it on the issues board. And thanks for making such a fantastic tool!

Start at the WebDriver::getLocator and WebDriver::getStrictLocator entry points and inspect how public functions such as WebDriver::findElements receive WebDriverBy arguments. Verify the module obtained through $this->getModule('WebDriver') can use both locator services from a custom action, and confirm invalid selector handling remains covered by the existing behavior shown in the issue.

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