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/appState.ts

@@ -32,6 +32,7 @@ export function getDefaultAppState(): AppState {
     zoom: 1,
     openMenu: null,
     lastPointerDownWith: "mouse",
+    selectedElementIds: {},
   };
 }