Use native FileDialog on OSX

Fix for issue #117 

Here are the pros/cons of FileDialog:
**Pros**
* Native - acts and looks better
* Supports keyboard shortcuts to jump to the file you want. Very nice on large filesystem

**Cons**
* No way to support the `Add file to current window` checkbox
* File filters do not work

The pros greatly outweigh the cons IMO. 
My implementation has the same behavior as if the checkbox were selected. That seems the most conservative.