In previous chapters I wrote why Layouts are not Widget. So what are they? They are pieces of common code that help to arrange SubWidgets within a parent Widget.
In order to avoid blocking entire Parent Widget while using &mut, layouts are in simple terms functions
F : ParentWidget -> [(pointer to subwidget, where to draw it)]
where pointer to subwidget is also a function F2 : ParentWidget -> Subwidget, or even more specifically a
pair of functions, because we might want to use mut or not.
This way layouts can be cloned (which is a cheap way to bribe the borrowchecker), stored etc, unbound to self of
parent Widget. This enables some flexibility while working on Bernardo, where I have not decided "how much the
Layouts will be responsible for" or "when are they being called". A complex Widget usually defines a Layout-building
function that returns an unbound Layout, and we can use it multiple times in render cycle.
There is some common-code for complex Widgets in ComplexWidget. It's not as mature as other parts of the library, but it serves it's purpose for some time now, so it might just stay around. I am not particularly proud of that part of design, I still hope for some kind enlightment for simplification.
I may introduce a simple DSL to build layouts at later time.