Plugin integration for extended functionality

[jvine]

Assuming features such as adding images to a canvas and modifying images on a canvas are not going to be part of initial Mirador 3.0 scope, it would be good to understand how a plugin designed to perform these functions would be seamlessly added into the application and how the design supports such extension points (without having to create a completely different layers dialog).

Originally posted by @beaudet in #6 (comment)