Add Random Button Functionality

Adds random pattern toward the center of the canvas. Does not delete anything.

lib/app_component.dart

@@ -48,5 +48,8 @@ class AppComponent implements OnInit {
     engine.reset();
   }
 
-  void onRandomClicked() {}
+  void onRandomClicked() {
+    engine.running = false;
+    engine.grid.addPattern();
+  }
 }

lib/src/Grid.dart

@@ -83,6 +83,7 @@ class Grid {
         }
         break;
     }
+    _dirty = true;
   }
 
   void setCellState(int x, int y, bool state) {