Prevent chained/duplicate calls to FancySelect.setValue

While debugging the setValue event handler I noticed the function is called twice; once for the originating event and once for the cascaded event that comes after the signal is dispatched. An abbreviated path of the event chain that causes the duplicate call is as follows:

```  
FancySelect:Dispatch(Change)  -> 
  editor:select -> editor:Dispatch(objectSelected) -> 
      SideBar.Scene:objectSelected -> 
        FancySelect.setValue()
```
This fix simply sets state tracking that a signal is being fired and then appropriately guards against calling setValue.