Add TextInput controlled selection prop on iOS

Summary:
This adds support for a controlled `selection` prop on `TextInput` on iOS (Android PR coming soon). This is based on the work by ehd in #2668 which hasn't been updated for a while, kept the original commit and worked on fixing what was missing based on the feedback in the original PR.

What I changed is:
- Make the prop properly controlled by JS
- Add a RCTTextSelection class to map the JS object into and the corresponding RCTConvert category
- Make sure the selection change event is properly triggered when the input is focused
- Cleanup setSelection
- Changed TextInput to use function refs to appease the linter

** Test plan **
Tested using the TextInput selection example in UIExplorer on iOS.
Also tested that it doesn't break Android.
Closes https://github.com/facebook/react-native/pull/8958

Differential Revision: D3771229

Pulled By: javache

fbshipit-source-id: b8ede46b97fb3faf3061bb2dac102160c4b20ce7

Libraries/Text/RCTTextSelection.h

@@ -0,0 +1,28 @@
+/**
+ * Copyright (c) 2015-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.
+ */
+
+#import "RCTConvert.h"
+
+/**
+ * Object containing information about a TextInput's selection.
+ */
+@interface RCTTextSelection : NSObject
+
+@property (nonatomic, assign, readonly) NSInteger start;
+@property (nonatomic, assign, readonly) NSInteger end;
+
+- (instancetype)initWithStart:(NSInteger)start end:(NSInteger)end;
+
+@end
+
+@interface RCTConvert (RCTTextSelection)
+
++ (RCTTextSelection *)RCTTextSelection:(id)json;
+
+@end