Clean up preference extensions/items a bit

core/src/main/java/tachiyomi/core/preference/Preference.kt

@@ -24,3 +24,16 @@ interface Preference<T> {
 }
 
 inline fun <reified T, R : T> Preference<T>.getAndSet(crossinline block: (T) -> R) = set(block(get()))
+
+operator fun <T> Preference<Set<T>>.plusAssign(item: T) {
+    set(get() + item)
+}
+
+operator fun <T> Preference<Set<T>>.minusAssign(item: T) {
+    set(get() - item)
+}
+
+fun Preference<Boolean>.toggle(): Boolean {
+    set(!get())
+    return get()
+}