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/util/utils.go

@@ -7,6 +7,7 @@
 package util
 
 import (
+	"fmt"
 	"net/url"
 	"reflect"
 	"sort"
@@ -70,6 +71,31 @@ func SetDefaults(data interface{}) error {
 	return nil
 }
 
+// CopyMatchingTag copies fields tagged tag:"value" from "from" struct onto "to" struct.
+func CopyMatchingTag(from interface{}, to interface{}, tag string, shouldCopy func(value string) bool) {
+	fromStruct := reflect.ValueOf(from).Elem()
+	fromType := fromStruct.Type()
+
+	toStruct := reflect.ValueOf(to).Elem()
+	toType := toStruct.Type()
+
+	if fromType != toType {
+		panic(fmt.Sprintf("non equal types: %s != %s", fromType, toType))
+	}
+
+	for i := 0; i < toStruct.NumField(); i++ {
+		fromField := fromStruct.Field(i)
+		toField := toStruct.Field(i)
+
+		structTag := toType.Field(i).Tag
+
+		v := structTag.Get(tag)
+		if shouldCopy(v) {
+			toField.Set(fromField)
+		}
+	}
+}
+
 // UniqueStrings returns a list on unique strings, trimming and sorting them
 // at the same time.
 func UniqueStrings(ss []string) []string {