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/types.ts

@@ -33,6 +33,7 @@ export type AppState = {
   zoom: number;
   openMenu: "canvas" | "shape" | null;
   lastPointerDownWith: PointerType;
+  selectedElementIds: { [id: string]: boolean };
 };
 
 export type Pointer = Readonly<{