Implement Popup Sizing and Placement

Sister Of https://github.com/adobe/brackets/pull/9394 

**CC:** @jasonsanjose @peterflynn 

Unfortunately I was not able to figure out how to place the Popup centered within the Application Frame on the Mac. I know there has to be a way to do it but all efforts were stifled.  There is a Cocoa API to Center the Window on the Display which seems to work fairly well so I'm using that for now.  I'll see if @bobeast has any thoughts on this but we may be stuck with just using the `center()` api

Windows works like a champ (as Bill Gates intended...)

No for Linux. No can do. The `CefWindowInfo` struct on Linux does not contain the `height` or `width` members so there is no easy way to implement this on Linux with CEF 1547. Hopefully when we upgrade to a newer version of CEF that will go away.  We might be able to hack something together using `gnome` APIs if @pthiess wants to invest the extra time to make that work but that would require quite a bit of hackery and deviation from the Mac/Windows code and I don't think it's worth the effort.  

I still have a little code-cleanup to do but wanted to see if you had time to test it and let me know what you think.
Since it's required that every platform implement this, shouldn't it be a pure virtual?

```
  virtual void ComputePopupPlacement(CefWindowInfo& windowInfo) = 0;
```