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/json.ts

@@ -14,7 +14,7 @@ export function serializeAsJSON(
       type: "excalidraw",
       version: 1,
       source: window.location.origin,
-      elements: elements.map(({ shape, canvas, isSelected, ...el }) => el),
+      elements,
       appState: cleanAppStateForExport(appState),
     },
     null,