1. In a computer system that supports a visual user interface development tool, a method of centrally managing user interface state information for the visual user interface development tool such that behavior for one or more user interface components or the visual user interface development tool itself may be defined dynamically at development time, the method comprising acts of: creating a transparent adorner window overlaying a design space of the visual user interface development tool, the transparent adorner window being adapted to intercept messages of the visual user interface development tool; intercepting a message generated within the visual user interface development tool during the development of one or more user interface components; sending the message to a centralized extensible behavior stack to check for one or more behaviors to use in processing the message, wherein a behavior describes the manner in which the user interface components or the visual user interface development tool processes the message resulting in a user interface activity dependent upon the behavior such that the functionality of the user interface components or the visual user interface development tool is defined by the behavior; checking the centralized extensible behavior stack containing currently available behaviors for processing messages to determine if a behavior is available to process the message; and if a behavior is available on the centralized extensible behavior stack, then passing the message to the available behavior for processing thereby resulting in a dynamically defined functionality and appearance of the interface; and wherein no behavior is available on the centralized extensible behavior stack for processing the message, the method further comprising an acts of: checking for a successfully hit tested glyph with a corresponding glyph behavior for the message; and if available, passing the message to the glyph behavior of the successfully hit tested glyph.