Extend MessageBox functionality

[frankbuckley]

A simple start was introduced in 36af919.

The idea is an easy-to use set of options for displaying modal dialogs - along the lines of the Win32 MessageBox function.

• Add a test/demo page
• Add information icon option
• Add Yes/No type (and corresponding results) - this will require second button, which should be styled secondary, probably want to extend Close() to take result value from button click binding
• Support cancellation - accept cancellation token into ShowAsync() (default = default) and run while loop in try {} catch {} catching OperationCanceledException, closing modal and rethrowing (this would allow callers to implement a timeout, for example)
• Add Retry/Cancel type (and corresponding results) - again, this requires second button

We should also properly implement Modal - e.g. ModalDialog, ModelContent, with ModalContent accomodating a ModalHeader, ModalBody and ModalFooter - which could then be used here - but that is perhaps a separate issue.