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/element/newElement.test.ts

@@ -54,8 +54,6 @@ it("clones arrow element", () => {
     ...element,
     id: copy.id,
     seed: copy.seed,
-    shape: undefined,
-    canvas: undefined,
   });
 });