Closing the widget should close affinder

If a user accidentally closes the affinder widget, the callbacks on the layers will stick around and it will be hard to remove them. It would be good if closing the widget was equivalent to stopping affinder. I don't know yet whether there is an event one can catch when the widget is closed, but there were some discussions about this either in the napari repo or in the Zulip.

No file or test is named. Start by locating the affinder widget lifecycle and the existing stop behavior, then check the referenced napari or Zulip discussions for how widget closure can be observed. Done means closing the widget has the same effect as stopping affinder and removes the layer callbacks.

Written by the indexing model from the issue text.