Add multi select & group nesting (#262)

* Add multi-select all canvas items (groups, nodes)

* Add Feat: Group & Node Multi-Select / Nesting

- Groups can now contain groups
- Nested groups re-order on top of parent groups
- Groups can be added / removed from selection
- Uses new Positionable interface - easily extensible to new types

* Enhance add / remove from selection UX

More in line with normal desktop UX.  Structured for keys to be customisable (if impl. later).

* Fix regression in link highlight

Legacy selection code still in use

* Allow nested groups to align perfectly on edges

* Remove group-move position rounding

Did not work under all circumstances, and resulted in misalignment more often than it helped.

src/interfaces.ts

@@ -12,6 +12,11 @@ export type NullableProperties<T> = {
 
 export type CanvasColour = string | CanvasGradient | CanvasPattern
 
+/** 
+ * An object that can be positioned, selected, and moved.
+ * 
+ * May contain other {@link Positionable} objects.
+ */
 export interface Positionable {
     id: NodeId | number
     /** Position in graph coordinates.  Default: 0,0 */