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/runtime.js

@@ -501,6 +501,14 @@ class Runtime extends EventEmitter {
         return 'PROJECT_LOADED';
     }
 
+    /**
+     * Event name for report that a change was made that can be saved
+     * @const {string}
+     */
+    static get PROJECT_CHANGED () {
+        return 'PROJECT_CHANGED';
+    }
+
     /**
      * Event name for targets update report.
      * @const {string}
@@ -1737,7 +1745,6 @@ class Runtime extends EventEmitter {
         // Script glows must be cleared.
         this._scriptGlowsPreviousFrame = [];
         this._updateGlows();
-        this.requestTargetsUpdate(editingTarget);
     }
 
     /**
@@ -2035,6 +2042,13 @@ class Runtime extends EventEmitter {
         this.emit(Runtime.PROJECT_LOADED);
     }
 
+    /**
+     * Report that the project has changed in a way that would affect serialization
+     */
+    emitProjectChanged () {
+        this.emit(Runtime.PROJECT_CHANGED);
+    }
+
     /**
      * Report that a new target has been created, possibly by cloning an existing target.
      * @param {Target} newTarget - the newly created target.