workspace: Simplify detecting added dialogs after closing a window
When trying to close a window in the overview by clicking the close
button and the window doesn't get closed but a dialog is added to the
window afterwards, we close the overview and show the dialog.
Instead of adding a separate listener for the window-added signal to the
WindowOverlay, let the WindowClones remember that the close button was
pressed and activate themselves if a dialog is added after that.