Packer abstraction

A re-implementation of Tk's default(?) geometry manager, the packer.

• Examples: A Packer example.
• Interface description: Overview of functionality. Packer Reference:
• Interface index: complete list of functionality provided.
• Creator index: how to create a Packer.
• Data type index: Data types introduced.
• Operator index: Operators on component.
• Resources: How to customise resources.