[Layout] Rename ASStackLayoutable to ASStackLayoutElement (#2322)

* redo all renaming, push new branch

* rename a few missed occurances of static

AsyncDisplayKit/Layout/ASAbsoluteLayoutElement.h

@@ -0,0 +1,25 @@
+//
+//  ASAbsoluteLayoutElement.h
+//  AsyncDisplayKit
+//
+//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+
+NS_ASSUME_NONNULL_BEGIN
+
+/**
+ *  Layout options that can be defined for an ASLayoutElement being added to a ASAbsoluteLayoutSpec.
+ */
+@protocol ASAbsoluteLayoutElement
+
+/**
+ * @abstract The position of this object within its parent spec.
+ */
+@property (nonatomic, assign) CGPoint layoutPosition;
+
+@end
+
+NS_ASSUME_NONNULL_END