Improvements in ASCollectionGalleryLayoutDelegate (#496)

* Improvements in ASCollectionGalleryLayoutDelegate
- It now can handle section inset, as well as interitem and line spacings
- Other small changes

* Fix build failure and update file licenses

* Update CHANGELOG

* Minor change

* Another assertion on scrollable directions of gallery layout delegate

Source/Private/_ASCollectionGalleryLayoutInfo.h

@@ -0,0 +1,30 @@
+//
+//  _ASCollectionGalleryLayoutInfo.h
+//  Texture
+//
+//  Copyright (c) 2017-present, Pinterest, Inc.  All rights reserved.
+//  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
+//
+
+#import <UIKit/UIKit.h>
+
+@interface _ASCollectionGalleryLayoutInfo : NSObject
+
+// Read-only properties
+@property (nonatomic, assign, readonly) CGSize itemSize;
+@property (nonatomic, assign, readonly) CGFloat minimumLineSpacing;
+@property (nonatomic, assign, readonly) CGFloat minimumInteritemSpacing;
+@property (nonatomic, assign, readonly) UIEdgeInsets sectionInset;
+
+- (instancetype)initWithItemSize:(CGSize)itemSize
+              minimumLineSpacing:(CGFloat)minimumLineSpacing
+         minimumInteritemSpacing:(CGFloat)minimumInteritemSpacing
+                    sectionInset:(UIEdgeInsets)sectionInset NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)init __unavailable;
+
+@end