Snap everything to grid (#315)

* Implement snap to grid

- Moves positioning logic to LGraph
- Simplifies code
- Adds Pointer API to alt-clone node
- Removes always_round_positions, replaced by always snap to grid (default size is 1 when always snapping)

Fix refator error

* Fix group items snapped without group

* Allow snapping of all items

- Add snapToGrid to Positionable
- Impl. on all types
- Deprecated: alignToGrid is now a wrapper

* Fix test import alias, update expectations

* Prevent desync of before / after change events

Adds ability to perform late binding of finally() during drag start.

* nit - Refactor

* Fix unwanted snap on node/group add

* nit - Doc

* Add shift key state tracking for snap to grid

Private impl., no state API as yet.

* Add snap guides rendering

Nodes, reroutes

* Optimisation - reroute rendering

 Fixes exponential redraw

* Add snap guidelines for groups

src/interfaces.ts

@@ -42,6 +42,15 @@ export interface Positionable extends Parent<Positionable> {
      */
     move(deltaX: number, deltaY: number, skipChildren?: boolean): void
 
+    /**
+     * Snaps this item to a grid.
+     * 
+     * Position values are rounded to the nearest multiple of {@link snapTo}.
+     * @param snapTo The size of the grid to align to
+     * @returns `true` if it moved, or `false` if the snap was rejected (e.g. `pinned`)
+     */
+    snapToGrid(snapTo: number): boolean
+    
     /**
      * Cached position & size as `x, y, width, height`.
      * @readonly See {@link move}