Mac: Unable to cycle between open windows.

Tested on Mac, 1.0alpha (8a5469a, 2019-05-29).

If more than one document is open, there is no way to switch between which is visible/in front. Standard Mac practice is to (1) have a "Window" menu that lists open documents, so that you can move one to the front, and (2) to allow you to cycle between which window is frontmost with ⌘-`.

Adding support for ⌘-` would be sufficient to close this issue.