[ASLayout] General Layout Cleanup (#2920)

* Remove duplicate import

* Move `ASLayoutController` related code out of CoreGraphics+ASConvenience

* Further cleanup

* Move private layout files to Privat/Layout

* Move ASLayoutElementStylePrivate into Private/Layout

* Further cleanup

* Move tvOS related files to tvOS folder

* Further cleanup

AsyncDisplayKit/Private/Layout/ASLayoutElementStylePrivate.h

@@ -0,0 +1,29 @@
+//
+//  ASLayoutElementStylePrivate.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.
+//
+
+#pragma once
+
+#import "ASObjectDescriptionHelpers.h"
+
+@interface ASLayoutElementStyle () <ASDescriptionProvider>
+
+/**
+ * @abstract The object that acts as the delegate of the style.
+ *
+ * @discussion The delegate must adopt the ASLayoutElementStyleDelegate protocol. The delegate is not retained.
+ */
+@property (nullable, nonatomic, weak) id<ASLayoutElementStyleDelegate> delegate;
+
+/**
+ * @abstract A size constraint that should apply to this ASLayoutElement.
+ */
+@property (nonatomic, assign, readonly) ASLayoutElementSize size;
+
+@end