[TASK] code cleanup: restructure interfaces

- create a new folder ```lib/Phile/Gateway/``` and namespace ```Phile\Gateway\``` for all global interfaces
- create a new folder ```lib/Phile/ServiceLocator/``` and namespace ```Phile\ServiceLocator\```
- move all service locator interfaces to the new location, rename it, mark it as deprecated and create wrapper interfaces for the old interfaces which will be removed in v1.0.0
- move all other interfaces into the new location and namespace

**Overview of old and new interfaces**

Old Interface | New Interface
------------- | -------------
```Phile\EventObserverInterface``` | ```\Phile\Gateway\EventObserverInterface```
```Phile\Cache\CacheInterface``` | ```\Phile\ServiceLocator\CacheInterface```
```Phile\Parser\MetaInterface``` | ```\Phile\ServiceLocator\MetaInterface```
```Phile\Parser\ParserInterface``` | ```\Phile\ServiceLocator\ParserInterface```
```Phile\Persistence\PersistenceInterface``` | ```\Phile\ServiceLocator\PersistenceInterface```
```Phile\Template\TemplateInterface``` | ```\Phile\ServiceLocator\TemplateInterface```

**IMPORTANT: the first interface (EventObserverInterface) is used by the most of all plugins**