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.
There isn't a need to derive from this class and implement a different version of it. That *would* require virtual.  It's just once per platform so it can be statically linked.

Point of reference   `PopupCreated` has to be implemented on every plattorm as well.