Experiment with using `cross_intrinsic_size` bookkeeping in Placer

[lucasmerlin]

…as a potential solution to #5138

what does desired_size mean?
in case of text: longest width of the child label, at most max_width

calculate label width from galley

• how does it work with ellipses?

Naming: Maybe desired_size?

[lucasmerlin]

I've been experimenting with this here: master...lucas/experiments/measure-widget-size

I think the biggest source of complexity here is correctly determining the "preferred size" in each widget, as the calculations in e.g. Button tend to get quite complex with support for right text, images, etc..

I propose we introduce a WidgetLayout struct that handles laying out arbitray things (text, images, custom shape) inside a widget. This would remove a lot of complexity from each Widgets code and also make it easier to implement custom 3rd party widgets.
Also widgets get more flexible, as it would be trivial to e.g. create a button with an image on the right.

The layout calculation should be pretty basic, only horizontal, only allow a single item to shrink / wrap, just enough to implement all the current widgets.

Calculation of the preferred_size could then be handled there transparently to each widget.

I made a prototype of this here:
master...lucas/experiments/widget_layout