Implement proper support for tags

[gurkenlabs]

Currently, an Entity in the engine already supports the possibility to provide tags (from code) and the Environment implementation already allows to find entities by a specified tag.

We need to extend the editor, such that it's possible to simply add tags to an IMapObject from the UI. For that, we need to introduce a new MapObjectProperty and implement a custom control (see screenshot).
Also, we need to implement something that makes the MapObjectLoaders aware of the tags and automatically adds them to the entity.

Remember to remove the deprecated IEnvrironment.getTriggers(string) method when this implementation is done.