Enable: add a component gallery to the documentation

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