Minor in JS lib: replaced LocalStorageClass with Storage with more accurate definitions and added sessionStorage.

#KT-6983 Fixed

js/js.libraries/src/html5/localstorage.kt

@@ -1,10 +0,0 @@
-package html5.localstorage
-
-native
-public val localStorage : LocalStorageClass = noImpl
-
-native
-public class LocalStorageClass() {
-    public fun getItem(key : String) : Any? = noImpl
-    public fun setItem(key : String, value : Any?) : Unit = noImpl
-}

js/js.libraries/src/html5/storage.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kotlin.js.dom.html5
+
+native
+public val localStorage: Storage = noImpl
+
+native
+public val sessionStorage: Storage = noImpl
+
+native
+public trait Storage {
+    public val length: Int = noImpl
+
+    public fun key(index: Int): String? = noImpl
+
+    native("getItem")
+    public fun get(key: String): String? = noImpl
+    native("setItem")
+    public fun set(key: String, value: String) {}
+
+    native("removeItem")
+    public fun remove(key: String) {}
+    public fun clear() {}
+}