This simplifies the rect calculations by making the gap between pixels
into a padding applied directly to the leaf nodes. This means the gaps
won't be present in the overlay layer, so when dragging a node around it
is always over an OverlayNode and the pendingAction won't be unset.
Also simplifies onDrop handling
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.
