Component based UI system

[davesmith00000]

The PR adds a new component based UI system to Indigo, allowing you to build UI's that automatically flow the layout. You can use it in two ways:

1. You can render components and groups of components yourself, anywhere. (Ugly demo included in this PR)
2. You can use the WindowManager subsystem to get window-like functionality. (Nice demos in the Roguelike-starterkit)

This work has been a long time coming. I think I've been looking at it off and on for nine months now, and I'm confident that it will still have a lot of sharp edges and areas that could be improved. That said, it feels like a decent foundation to build on and is surely useful as-is, even if imperfect. Also I'm very pleased to see the back of it!

This work started life in the roguelike-starterkit, but has been made generic and backported to Indigo.

For now, the outstanding work is that we're going to need demos and docs, but that should live in indigo-docs. I have tested all this in the Roguelike-starterkit, and will do so again before merging this PR.

This PR also moves the existing Indigo UI components from indigoextras.ui.* to indigoextras.ui.simple.*.

The new work can be imported with:

import indigoextras.ui.*
import indigoextras.ui.syntax.*

switches_and_radios.mov

switches_and_radios_terminal.mov

[davesmith00000]

Updated the roguelike-starterkit PR and it all works fine when published locally (CI will have to wait for an Indigo release).

[davesmith00000]

I'm going to go ahead and merge this so that we've got time to try using it before we release it.