Add standard way to get font out of NUISettings.
Use consistent font retrieval throughout code
Support system, boldSystem, and italicSystem for font names.
Move font color handling out of renderText.

Without this change, UIButton handles the `font-color` and `font-color-highlighted` properties twice, once per state and once for the label, which can cause problems, especially for UIButton subclasses.
Merge pull request #214 from phatmann/font-from-settings

Add standard way to get font out of NUISettings.