Fixed: unable to chain sheet in a window

Previously when chaining sheet on the same window, Cappuccino crashed.
Now chaining a sheet in the delegate method of the method endSheet: will open the next sheet.

Now when opening a sheet, we check if another sheet is currently closing. If yes, we wait till we get the notification of the closing of the previous sheet. And then we open the next sheet.

Fixed #2159

Test app in Tests/Manual/AttachedSheet2/SheetWindowController.j