Reduce redundant layout calculations

### What is the problem or limitation you are having?

Previous discussion:
* https://github.com/beeware/toga/pull/1794#issuecomment-1453096289
* https://github.com/beeware/toga/issues/2937#issuecomment-2449761045

Except for WinForms, none of the native platforms make any UI changes visible until the next main loop iteration, so we're doing a lot of redundant work. And even on WinForms, excessive updates will make the UI look unstable, and probably reduce performance.

This is particularly an issue with layout – if you insert N items into the layout, or you have N labels and you change the font or text on all of them, then we'll perform N layout passes, which will cost O(N ** 2) time.

### Describe the solution you'd like

Delay layout updates until the end of the current main loop iteration.

This may also affect non-layout style properties if they're assigned multiple times within a single main loop iteration, where only the final value really matters. But that's likely to be less of a performance issue, so it might not be worth the effort.

Start by reading the prior discussion in PR #1794 and issue #2937, since no implementation files or tests are named here. Trace how layout updates are scheduled across the native platforms, then verify that multiple changes in one main-loop iteration produce one effective layout pass without making WinForms updates unstable.

Written by the indexing model from the issue text.