Reduce redundant layout calculations

[mhsmith]

What is the problem or limitation you are having?

Previous discussion:

• Modify container handling for GTK #1794 (comment)
• Restructure widget initialization #2937 (comment)

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.

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 loopiteration.

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.