New render protocol

[willmcgugan]

This adds a new system for rendering content. Previously Textual uses Rich Renderables.

Now we have a textual.visual.Visual which is a replacement for the renderable protocol, which allows for layered alpha colors via the Content class.

This is backwards compatible in that Textual you can still return a renderable via the render() method, which will be wrapped in a textual.visual.RichVisual class.

This is a large change, but only the OptionList actively uses Visuals currently.

In the future, there will be a Visual markup language, like console markup, but supporting alpha transparency and CSS variables. For instance, you could do this:

return "[$accent on blue %20]Hello, World![/]"

We don't have that yet, and there are still gaps in functionality, so this will remain undocumented for now.