Editable document performance improvements

[matthew-carroll]

Fix various performance issues with document editing.

Use-Cases

From @brian-superlist - 20-ish page Document that contains various paragraphs and images for testing, was rather sluggish on my machine in release mode and had high CPU usage (bad for batteries): https://github.com/brian-superlist/super_editor/blob/use-cases/super_editor/example/lib/demos/scrolling_performance_use_case.dart

Funders

Superlist

[matthew-carroll]

I closed a ticket in favor of this milestone: #309