More Docs + Code to explain UI usage.

I still find creating UI's in blender quite confusing. I'm getting code examples from blender working, but it's tricky to understand.

E.g. How do you create a UI for editing "Object" properties?
- UI for opening a file and linking to an object, and that shows files linked to an object.
- UI which affect the blender envrionment, which affect "Scenes", "tools"... etc.