gui, lib/config, lib/model: Support auto-accepting folders (fixes #2299)

Also introduces a new Waiter interface for config changes and segments the
configuration GUI.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4551

lib/config/config_test.go

@@ -826,7 +826,7 @@ func TestSharesRemovedOnDeviceRemoval(t *testing.T) {
 		t.Error("Should have less devices")
 	}
 
-	err = wrapper.Replace(raw)
+	_, err = wrapper.Replace(raw)
 	if err != nil {
 		t.Errorf("Failed: %s", err)
 	}