network/connection-editor: destroy the editor when closing

If an user opens an connection editor and then closes it, repeat this
multiple times causes lots of unused hidden editor windows.

Fix that by destroying the connection editor instead of hiding them
when closing.

panels/network/connection-editor/net-connection-editor.c

@@ -81,7 +81,7 @@ static void page_changed (NetConnectionEditor *self);
 static void
 cancel_editing (NetConnectionEditor *self)
 {
-        gtk_widget_hide (GTK_WIDGET (self));
+        gtk_window_destroy (GTK_WINDOW (self));
         g_signal_emit (self, signals[DONE], 0, FALSE);
 }