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/history.ts

@@ -18,10 +18,8 @@ export class SceneHistory {
   ) {
     return JSON.stringify({
       appState: clearAppStatePropertiesForHistory(appState),
-      elements: elements.map(({ shape, canvas, ...element }) => ({
+      elements: elements.map(element => ({
         ...element,
-        shape: null,
-        canvas: null,
         points:
           appState.multiElement && appState.multiElement.id === element.id
             ? element.points.slice(0, -1)