Flesh out ImageJ 2.0 class hierarchy

[curtis]

This ticket is a blanket one covering the general redesign of the core ImageJ public API, including:

• image data model
• metadata model
• I/O
• user interface
• plugins
• scripting
• undo history
• events
• add-ons

By mid-year, we plan to have a rough draft of the revised API, at least at the package level, with most classes defined.

[curtis]

The design has come a long way, with many of the issues above sorted. The  web site discusses top-level details, and project organization.

A few of the issues above still need to be resolved but since they each have separate tickets, I'm closing this one.