Refactor ExcalidrawElement (#874)

* Get rid of isSelected, canvas, canvasZoom, canvasOffsetX and canvasOffsetY on ExcalidrawElement.

* Fix most unit tests. Fix cmd a. Fix alt drag

* Focus on paste

* shift select should include previously selected items

* Fix last test

* Move this.shape out of ExcalidrawElement and into a WeakMap

src/data/restore.ts

@@ -57,10 +57,6 @@ export function restore(
             ? 100
             : element.opacity,
         points,
-        shape: null,
-        canvas: null,
-        canvasOffsetX: element.canvasOffsetX || 0,
-        canvasOffsetY: element.canvasOffsetY || 0,
       };
     });