Add demo pages for context menus, dialogs, and the clipboard, and fix what they found.

[cmeyer]

• Report the outcome of an ok cancel dialog once.
• Inset the content of the ok cancel and action dialogs, buttons included.
• Unwrap a canvas window before it reaches the host user interface.
• Report where a canvas widget is on the screen.
• Answer the file dialogs and an empty clipboard in the test user interface.
• Add context menu, dialog, and clipboard demo pages to the ui demo application.