demo Angular - updated code to Ang13+

* ComponentFactoryResolver is no longer needed
* select="[empty-content]" *ngIf="isEmpty" is cleaner
* added bug caveat to readme.

Author: adumesny

demo/angular/src/app/README.md

@@ -83,6 +83,7 @@ to build the demo, go to demo/angular and run `yarn` + `yarn start` and Navigate
 
  - This wrapper needs v7.2+ to run as it needs the latest changes
  - Code isn't compiled into a lib YET. You'll need to copy those files. Let me know (slack) if you are using it...
+ - BUG: content doesn't appear on new widget until widget is moved around (or another created that pushes it). Need to force angular detection changes...
 
  ## ngFor Caveats
  - This wrapper handles well ngFor loops, but if you're using a trackBy function (as I would recommend) and no element id change after an update,

demo/angular/src/app/app.component.ts

@@ -52,6 +52,8 @@ export class AppComponent {
    * TEST dynamic grid operations - uses grid API directly (since we don't track structure that gets out of sync)
    */
   public add(gridComp: GridstackComponent) {
+    // TODO: BUG the content doesn't appear until widget is moved around (or another created). Need to force
+    // angular detection changes...
     gridComp.grid?.addWidget({x:3, y:0, w:2, content:`item ${ids}`, id:String(ids++)});
   }
   public delete(gridComp: GridstackComponent) {