Update UIWidget, DataView* widgets to use SDL2Logger categories

* Introduce NOM_LOG_TRACE_PRIO macro.
Update documentation files

* Add demo URL to VisualUnitTest's
README.
* Relocate SDL2Logger's TODO item
regarding implementing color output to
https://github.com/i8degrees/nomlib/issues
Fix compile errs in engine examples

* RenderWindow::size was recently updated to return a Size2i
object.