Add Support for 2D-Billboards

Add the possibility to add 2D-Billboard to the 3D-Scene.

As first step that could be a text, but later also just a plane texture.

That support should be build in into the runtime as core feature. I can imagine that the text element has an additional property where this option can be set.

That billboard can be then easily attached to another element, it will keep its 2D size, but will move with it in all directions.