Fix backup/restore of category related preferences (#1726)

Co-authored-by: AntsyLich <59261191+AntsyLich@users.noreply.github.com>
(cherry picked from commit e1724d1aa0e3340e1404cfd80bd264831d86a879)

# Conflicts:
#	CHANGELOG.md

core/common/src/main/kotlin/tachiyomi/core/common/preference/Preference.kt

@@ -57,6 +57,10 @@ operator fun <T> Preference<Set<T>>.plusAssign(item: T) {
     set(get() + item)
 }
 
+operator fun <T> Preference<Set<T>>.plusAssign(items: Iterable<T>) {
+    set(get() + items)
+}
+
 operator fun <T> Preference<Set<T>>.minusAssign(item: T) {
     set(get() - item)
 }