[5.5] Add option to get an route action by key

[ThorbenG]

With this PR its easier to get an specific route action.

Before you had to use the getAction() method to get the action array from a route and than use the key to get an action like this:
$route->getAction()['controller'];

Now you can get the action directly by using:
$route->getAction('controller');

Without an valid action key the method returns the action array.

[kamui545]

Pretty cool. However, am I the only one bothered by the fact that it is returning the action array if the key is not found?

Wouldn't it be more appropriate to return null instead as we expect a very specific property?

I find this misleading, especially if using it in a condition.
Eg: if ($route->getAction('controller')), even if there is no controllers it will return true...