Add a MessageDialog for Web projects

Add a MessageDialog to Web projects so applications can ask the user a modal question or present important information in-page — for example confirming a destructive action, reporting a completed operation, or choosing between alternatives.

The dialog should show a title, message, and optional explanation, an optional icon, and up to three buttons with application-defined captions, and tell the application which button the user chose. This complements the existing notification surfaces (which are transient and non-interactive) with a blocking-style interaction, and brings Web projects to parity with the desktop MessageDialog for confirmation workflows.