This fixes an issue where pressing the red close button in a window or
the "x" button on a tab couldn't differentiate and would always close
the tab or close the window (depending on tab counts).
It seems like in both cases, AppKit triggers the `windowShouldClose`
delegate method on the controller, but for the close window case it
triggers this on ALL the windows in the group, not just the one
that was clicked.
I implemented a kind of silly coordinator that debounces
`windowShouldClose` calls over 100ms and uses that to differentiate
between the two cases.
