Enable: add a component gallery to the documentation

[corranwebster]

There are a number of useful components that are provided by Enable, and there should be a basic list of them, their purpose, and the key traits, etc. for controlling their behaviour. This includes:

• compass
• controls, slider, radio group, scrollbar (maybe, not sure if they work, or that we want to highlight them!)
• label
• native scrollbar
• scrolled
• text field
• text field grid
• text grid
• viewports, canvas and viewables
• gadgets/vu_meter
• primitives/* (but particularly image)
• drawing/* (maybe, I think that they work)
• the various container types

[rahulporuri]

Some of the above components are more important to end users/devs than others so we will first sort the above list based on priority/impact and them address them in the documentation.