Allow launching a single level breakpoint split view focused on the breakends

[cmdcolin]

This adds a number of fixes including an option to create "focused" single-level breakpoint split view. This zooms in on just the breakends

it also has a couple rider changes including

• Unified logic for launching multi-level breakpoint split view (several different variants were active in the codebase)
• Allows mouseovering breakends and alignment squiggles from breakpoint split view, and still allow interaction with the rest of the app. this is a clever use of pointerEvents:'auto' inside of a pointerEvents:'none' container (so the children allow pointer events like mouseover, the container does not)