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/scene/index.ts

@@ -1,6 +1,5 @@
 export { isOverScrollBars } from "./scrollbars";
 export {
-  clearSelection,
   getSelectedIndices,
   deleteSelectedElements,
   isSomeElementSelected,