Introduce Shapelock (#480)

* Introduce shape lock

* Format code with prettier

* Do not reset elementLocked on selection change

* Don't set isSelected to true if element is locked

* Don't reset the cursor

* Move reset cursor call to better spot

* Run prettier + lint

src/types.ts

@@ -7,6 +7,7 @@ export type AppState = {
   //  (e.g. text element when typing into the input)
   editingElement: ExcalidrawElement | null;
   elementType: string;
+  elementLocked: boolean;
   exportBackground: boolean;
   currentItemStrokeColor: string;
   currentItemBackgroundColor: string;