Allows a component to support a font context property.
The property will be set on the component by the framework
as the child is added to the display list.
A font context is important for components that create flash.text.TextField
objects with embedded fonts.
If an embedded font is not registered using Font.registerFont(),
TextField objects can only use embedded fonts if they are created
in the context of the embedded font.
This interface provides for tracking the font context of a component.