This was hard to debug. It manifested in nodes that had been collapsed
into their parent. When a node was dragged over the recently-collapsed
node, the placeholder would show the action as if the flex direction was
reversed. It turns out useDrag has trouble tracking changes to the
LayoutNode objects. For now, I am just finding the nodes again in the
computeMoveNode function. Later when I redo the drag system, I will fix
this better.
This PR is a large refactoring of the layout code to move as much of the
layout state logic as possible into a unified model class, with atoms
and derived atoms to notify the display logic of changes. It also fixes
some latent bugs in the node resize code, significantly speeds up
response times for resizing and dragging, and sets us up to fully
replace the React-DnD library in the future.
