Add article on type visualizers and visualizer composition

[glopesdev]

One of the aspects that remains largely undocumented is type visualizers: how they work, how they are implemented, and more recently how they can be combined with each other using visualizer mappings and visualizer layouts.

There is a lot to cover here, in no particular order:

• Mashups
• TableLayouts
• Using subjects to build self-contained visualizers
• Select amongst available visualizers