[ASLayoutSpec] Initial commit to support visualizing layout specs (with Playground app). (#2554)

* Initial ASLayoutSpecPlayground commit

* Initial exploratory stab at the main challenge of the app - visualizing ASLayoutSpecs

* Halfway through moving debug features out of ASDK framework files and into debug files. Project builds.

* [ASLayoutSpecPlayground] Created new Inspector node, cleaning up internal implementation to start formalizing support for layout spec visualization.

* Workaround for ensuring creation of visualizerNode for ALL layoutspecs

* continued development

* Layout Inspector Work in Progress

* Resizing the playground works in the shrink direction, not for grow.

* added new ASLayoutableInspectorNode features

* Cleaned up examples code.

* Cleaning up  code.

* more code cleanup

* [ASLayoutableInspector] Transition to an ASTableNode-based architecture to support larger numbers of buttons / customizable types.

* [ASLayoutableInspector] Support different layoutable property types to set up buttons that can edit all of them.

* Huy debugging

* Refactored layout inspector code for extensibility.

* Properly lock layoutableContextMap

* Fix context handling in ASDisplayNode:measureWithSizeRange

* Fix ASLayoutSpecPlayground:ViewController:toggleVisualization

* added slider to InspectorCell

* [ASLayoutSpecPlayground] Improvements to propagation of visualize mode, resize handle, minor cleanup.

* Fix to ASEnvironment

* [ASLayoutSpecPlayground] Fix a few minor issues from the merge with latest master.

* Implement layout spec cache

* add pager ndoe

* add more examples

* add more layout examples

* [ASLayoutPlayground] Fix merge issues

* [ASLayoutPlayground] Fix up the example project from the 2.0 API changes.

* [ASLayoutPlayground] Some fixes (#2411)

* [ASLayoutPlayground]: Some fixes
* Fixed crash when tapping descender.
* Fixed setting the item to inspect.
* Fixed button states in inspector node.
* Added sliders for spacingBefore, spacingAfter, ascender.

* [ASLayoutSpecPlayground] Deselect the buttons when editing is over.

* [ASLayoutSpecPlayground] Changed flexGrow/Shrink's values from YES/NO to 1.0/0.0

* [Project] Create new Debug/ directory for advanced tools dedicated to debugging.

* [LayoutSpecPlayground] Rename project without AS in title, to be consistent with LayoutSpecExamples.

* [Bulid] Fix Xcode project to use new Debug subdirectory / group.

* [Bulid] Fix a small merge error.

* [Build] Fix build issue for Framework target.

* [Bulid] Fix podspec to expose InspectorNode header; Remove old-cocoapods emojis from ASDKgram :)

* Move aside ASLayoutSpecPlayground-Swift to match master

* [LayoutSpecPlayground] Cleanup implementation in several files, xcodeproj, etc.

* [ASControlNode] Add comment for new assertion, to be enabled in a separate diff.

AsyncDisplayKit/Debug/ASLayoutElementInspectorCell.h

@@ -0,0 +1,28 @@
+//
+//  ASLayoutElementInspectorCell.h
+//  AsyncDisplayKit
+//
+//  Created by Hannah Troisi on 3/27/16.
+//  Copyright © 2016 Facebook. All rights reserved.
+//
+
+#import <AsyncDisplayKit/ASCellNode.h>
+
+typedef NS_ENUM(NSInteger, ASLayoutElementPropertyType) {
+  ASLayoutElementPropertyFlexGrow = 0,
+  ASLayoutElementPropertyFlexShrink,
+  ASLayoutElementPropertyAlignSelf,
+  ASLayoutElementPropertyFlexBasis,
+  ASLayoutElementPropertySpacingBefore,
+  ASLayoutElementPropertySpacingAfter,
+  ASLayoutElementPropertyAscender,
+  ASLayoutElementPropertyDescender,
+  ASLayoutElementPropertyCount
+};
+
+@interface ASLayoutElementInspectorCell : ASCellNode
+
+- (instancetype)initWithProperty:(ASLayoutElementPropertyType)property layoutElementToEdit:(id<ASLayoutElement>)layoutable NS_DESIGNATED_INITIALIZER;
+
+@end
+