Add "project changed" event

And emit it whenever we think the project has changed. Try to not emit it when a change has happened internally that shouldn't affect the serialized project.

This iteration fires the event too frequently, e.g., when switching sprites. This is meant as a simple initial implementation that can be improved.

src/engine/blocks.js

@@ -487,6 +487,10 @@ class Blocks {
             }
             break;
         }
+
+        // forceNoGlow is set to true on containers that don't affect the project serialization,
+        // e.g., the toolbox or monitor containers.
+        if (optRuntime && !this.forceNoGlow) optRuntime.emitProjectChanged();
     }
 
     // ---------------------------------------------------------------------