Make target an integral part of cameras and lights

[ZieIony]

• make a new collection with support for being read-only
• make entity collection for cameras and lights read-only

[ZieIony]

Keeping target as a separate entity doesn't seem to be the best idea. Target contains useless attributes (like scale and rotation), doesn't make sense as a resource and will be hard to manage in hierarchical structure of scene objects. Making it internal will also solve the issue of being a read-only child.