Docs: Window / View section

[einarf]

We need a Window and View section going trough the basics. There's quite a few sections we can keep building out here. Events, window properties. fullscreen/windowed. viewport/scissor...

Explaining when and why to use a View is also a good idea.

[pushfoo]

Agreed. We're running into confusion over the on_ event methods, and I think we need to explain them better, especially:

1. on_show_view
2. on_hide_view
3. View + UIView
   We currently link this in the UI concepts.rst, but that also needs some cleaning.