Include more helper methods.

Hi,

I always felt that the Xenko engine was a bit lacking when it came some general helper methods. So I started the XenkoToolkit project but I really feel a bunch of these things should be included in-the-box. For example:

• Find components in entity hierarchies.
• Transforms using the Entity's Transform.
• Transforming things between world and screen space.
• Calculating rays for doing picking using physics ray cast.
• Transforming Prefabs when being instantiated.

I am creating this issue more as discussion. I am not super confident in the quality of my code as they have not been thoroughly tested and my math skills are not great. There are probably also a few questionable design decisions. For example, not sure about all the overloads I did either, may have gone a bit overboard. There are quite a few shortcomings as well, i.e. a bunch of the transform extensions throw exceptions if TransformComponent.UseTRS is false (I couldn't get the math right).

There are a number of things I would do differently if they were included in the engine, i.e. put the Quaterion look at function in the Quaternion struct.

There are somethings I would almost certainly leave out like the material stuff I did. A better option would be to change the ModelComponent and related systems to handle being able to set material properties per ModelComponent.

cheers,
dfkeenan