Remove references to windows when they close so that they will deallocate.

Mac/AppDelegate.swift

@@ -383,6 +383,12 @@ class AppDelegate: NSObject, NSApplicationDelegate, NSUserInterfaceValidations,
 		}
 	}
 
+	func removeMainWindow(_ windowController: MainWindowController) {
+		if let index = mainWindowControllers.firstIndex(of: windowController) {
+			mainWindowControllers.remove(at: index)
+		}
+	}
+	
 	// MARK: NSUserInterfaceValidations
 	func validateUserInterfaceItem(_ item: NSValidatedUserInterfaceItem) -> Bool {
 		if shuttingDown {

Mac/MainWindow/MainWindowController.swift

@@ -462,7 +462,9 @@ extension MainWindowController: NSWindowDelegate {
 	}
 
 	func windowWillClose(_ notification: Notification) {
+		// TODO: now that we are deallocating main window controllers, the media playback stop can probably be removed
 		detailViewController?.stopMediaPlayback()
+		appDelegate.removeMainWindow(self)
 	}
 	
 }