Add ability to edit custom UI control properties in the visual designer's inspector

If a custom UI control is created (e.g. a subclass of TextArea) then any properties added to that subclass should be accessible in the Visual Designer's Inspector when an instance of that control is added to a window.

We need an interface designed to all the creator of the subclass to decide how the properties of a custom control are presented in the inspector (as the developer may not intend for them all to be visible in the inspector).