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/showSelectedShapeActions.ts

@@ -8,6 +8,6 @@ export const showSelectedShapeActions = (
 ) =>
   Boolean(
     appState.editingElement ||
-      getSelectedElements(elements).length ||
+      getSelectedElements(elements, appState).length ||
       appState.elementType !== "selection",
   );