Separate widget logic/state and rendering

[rafern]

This allows for custom renderers, which might be better suited for the engine being used. For example:

• text is usually the most noticeably low-res part of the UI
• using mesh-based text when it's available (for example, in Wonderland Engine) would improve text quality a lot
• however, that would mean that a custom version of the Label widget would have to be written to support Wonderland Engine

If rendering is separated from widget state/logic, then this improvement could be done while keeping the base library engine-agnostic, and not having to make a custom version of each widget that uses text just for Wonderland Engine