Scene Actions callbacks

[Max-ChenFei]

this.onMouse = null;
this.onDrawOverlay = null; //to render foreground objects not affected by transform (for GUIs)
this.onNodeMoved = null; //called after moving a node
this.onSelectionChange = null; //called if the selection changes
this.onConnectingChange = null; //called before any link changes
this.onBeforeChange = null; //called before modifying the graph
this.onAfterChange = null; //called after modifying the graph