lib/model, lib/config: Support "live" device removal, folder unsharing and folder configuration changes

Furthermore:
1. Cleans configs received, migrates them as we receive them.
2. Clears indexes of devices we no longer share the folder with

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3478

lib/config/wrapper.go

@@ -136,6 +136,10 @@ func (w *Wrapper) Replace(cfg Configuration) error {
 func (w *Wrapper) replaceLocked(to Configuration) error {
 	from := w.cfg
 
+	if err := to.clean(); err != nil {
+		return err
+	}
+
 	for _, sub := range w.subs {
 		l.Debugln(sub, "verifying configuration")
 		if err := sub.VerifyConfiguration(from, to); err != nil {