Merge pull request #305 from UnityTech/modify_page_view

Modify page view a bit for implementation of swiper.

Runtime/widgets/page_view.cs

         public readonly float viewportFraction;
 
 
-        public float page {
+        public virtual float page {
             get {
                 D.assert(this.positions.isNotEmpty(),
                     () => "PageController.page cannot be accessed before a PageView is built with it."

Runtime/widgets/scroll_position.cs

 
         float? _maxScrollExtent;
 
+        public bool hasMinScrollExtent {
+            get { return this._minScrollExtent != null; }
+        }
+
+        public bool hasMaxScrollExtent {
+            get { return this._maxScrollExtent != null; }
+        }
+
         public override float pixels {
             get {
                 D.assert(this._pixels != null);