Extension template events are not loaded from subdirectories

In writing template event listeners for extensions so far, it has been the habit to place the files in the styles/[style]/template/events/ directory. Since the switch to twig as a template engine, however, template event listeners placed in this directory do not work anymore. They have to be placed directly in styles/[style]/template/.

Now, the old template engine searched all subdirectories recursively for template event files. This is probably not needed; in order to match the behavior of php event listeners, just searching styles/[style]/template/event/ for files ending in _listener.html is enough. I therefore propose to add 'event/' to the template event listener search path so it will only scan that subdirectory, and a '_listener' suffix to the location to pick up files in the same manner as with php event listeners.