Support easy loading of textures from an asset catalog

Since Imagine Engine doesn't rely on the default system image loading mechanism (using `UIImage(named:)` on iOS) loading textures from an asset catalog is not super easy. While *possible* it requires you to create `Animation` or `Texture` instances using `Image`, like this:

```swift
let image = Image(named: "player")!
actor.animation = Animation(image: image)
```

It would be nicer if Imagine Engine could automatically search the app's asset catalog for an image with a given name, so that you could simply use:

```swift
actor.animation = Animation(textureNamed: "player")
```