Alternative version of callback-taking functions for making API bindings

I'm working out how to wrap egui's API for my scripting language. Something that would make this a lot easier is if there was an alternative API to the functions that take callbacks (like `SidePanel::show`, `Window::show`, and `Ui::horizontal`) which could be written in straight-line code. For example, an object that exposes a `&mut UI` to draw widgets, and has a `finish` method which drops it.

It looks like I can emulate this with some pairs of `Ui::new` and `Ui::allocate_rect`, but I'll have to rewrite all the useful stuff those functions do.

Start by examining the callback-taking APIs named in the issue: SidePanel::show, Window::show, and Ui::horizontal. Compare their behavior with the mentioned Ui::new and Ui::allocate_rect approach, then determine the lifecycle and ownership requirements for an object exposing &mut Ui with a finish method. Done means a coherent alternative API that supports equivalent straight-line widget construction.

Written by the indexing model from the issue text.