I've created the FooRepresentable family of protocols now. Next I'll working on the FooHostingView family of views and then I'll come back and try to improve the experience of using the FooRepresentable protocols. The naming of the protocol requirements currently differ in just enough ways to be annoying when copy and pasting an existing implementation to adapt it for a new backend. Some suggestions were given in the Discord server that I'm considering going with.

The hardest part of implementing the FooRepresentable family of protocols was integrating with the layout system of the underlying native frameworks. Each framework exposes a different measurement API with unique limitations. The WinUI one is probably the most fragile because I had to apply a similar hack to the one used to get natural sizing working correctly; any WinUI elements that SwiftCrossUI itself doesn't already use are likely to have incorrect sizes before the first render. The other dodgy implementation is the GtkBackend one. It seems to work well enough, but throws a bunch of warnings because we occasionally measure with less height/width than the underlying widget can work with. I've searched around for APIs that would tell us what these minimum workable dimensions are, but I haven't found any yet. The issue is that when measuring you provide one dimension and get minimum/natural dimensions for the other dimension, but Gtk never actually tells you how much of the provided dimension it wants to use. Gtk 3 doesn't seem to care.

I've rebased this PR onto main now, which required a lot of tweaking within the ...Representable family of protocols, particularly in regards to their default sizeThatFits implementations.

I'm going to leave the ...HostingView family of structs for a future pull request in the interest of getting the existing improvements merged and moving on to other important work.