[TEST ONLY] 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.
I should have posted the `cef_window_info_t` struct.  

```c
typedef struct _cef_window_info_t {
  // Pointer for the parent GtkBox widget.
  cef_window_handle_t parent_widget;

  // If window rendering is disabled no browser window will be created. Set
  // |parent_widget| to the window that will act as the parent for popup menus,
  // dialog boxes, etc.
  bool window_rendering_disabled;

  // Set to true to enable transparent painting.
  bool transparent_painting;

  // Pointer for the new browser widget.
  cef_window_handle_t widget;
} cef_window_info_t;
```