MenuButton abstraction

A MenuButton is the activator for a pulldown menu.

• Examples: How to create a MenuButton.
• Interface description: Functionality provided. MenuButton Reference:
• Interface index: complete list of functionality provided.
• Creator index: Functions for creating a MenuButton.
• Operator index: Operators on component.
• Resources: How to customise resources.
• See also: Abstractions related to MenuButton.
• ToDo: Some unresolved MenuButton issues.