Implement CSS z-index for iOS

Summary:
This diff implement the CSS z-index for React Native iOS views. We've had numerous pull request for this feature, but they've all attempted to use the `layer.zPosition` property, which is problematic for two reasons:

1. zPosition only affects rendering order, not event processing order. Views with a higher zPosition will appear in front of others in the hierarchy, but won't be the first to receive touch events, and may be blocked by views that are visually behind them.
2. when using a perspective transform matrix, views with a nonzero zPosition will be rendered in a different position due to parallax, which probably isn't desirable.

See facebook/react-native#7825 for further discussion of this problem.

So instead of using `layer.zPosition`, I've implemented this by actually adjusting the order of the subviews within their parent based on the zIndex. This can't be done on the JS side because it would affect layout, which is order-dependent, so I'm doing it inside the view itself.

It works as follows:

1. The `reactSubviews` array is set, whose order matches the order of the JS components and shadowView components, as specified by the UIManager.
2. `didUpdateReactSubviews` is called, which in turn calls `sortedSubviews` (which lazily generates a sorted array of  `reactSubviews` by zIndex) and inserts the result into the view.
3.  If a subview is added or removed, or the zIndex of any subview is changed, the previous `sortedSubviews` array is cleared and  `didUpdateReactSubviews` is called again.

To demonstrate it working, I've modified the UIExplorer example from facebook/react-native#7825

Reviewed By: javache

Differential Revision: D3365717

fbshipit-source-id: b34aa8bfad577bce023f8af5414f9b974aafd8aa

Author: nicklockwood

UIExplorer/ViewExample.js

@@ -1,4 +1,11 @@
 /**
+ * Copyright (c) 2013-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.
+ *
  * The examples provided by Facebook are for non-commercial testing and
  * evaluation purposes only.
  *
@@ -30,7 +37,13 @@ var styles = StyleSheet.create({
     backgroundColor: '#527FE4',
     borderColor: '#000033',
     borderWidth: 1,
-  }
+  },
+  zIndex: {
+    justifyContent: 'space-around',
+    width: 100,
+    height: 50,
+    marginTop: -10,
+  },
 });
 
 var ViewBorderStyleExample = React.createClass({
@@ -74,6 +87,53 @@ var ViewBorderStyleExample = React.createClass({
   }
 });
 
+var ZIndexExample = React.createClass({
+  getInitialState() {
+    return {
+      flipped: false
+    };
+  },
+  
+  render() {
+    const indices = this.state.flipped ? [-1, 0, 1, 2] : [2, 1, 0, -1];
+    return (
+      <TouchableWithoutFeedback onPress={this._handlePress}>
+        <View>
+          <Text style={{paddingBottom: 10}}>Tap to flip sorting order</Text>
+          <View style={[
+            styles.zIndex,
+            {marginTop: 0, backgroundColor: '#E57373', zIndex: indices[0]}
+          ]}>
+            <Text>ZIndex {indices[0]}</Text>
+          </View>
+          <View style={[
+            styles.zIndex,
+            {marginLeft: 50, backgroundColor: '#FFF176', zIndex: indices[1]}
+          ]}>
+            <Text>ZIndex {indices[1]}</Text>
+          </View>
+          <View style={[
+            styles.zIndex,
+            {marginLeft: 100, backgroundColor: '#81C784', zIndex: indices[2]}
+          ]}>
+            <Text>ZIndex {indices[2]}</Text>
+          </View>
+          <View style={[
+            styles.zIndex,
+            {marginLeft: 150, backgroundColor: '#64B5F6', zIndex: indices[3]}
+          ]}>
+            <Text>ZIndex {indices[3]}</Text>
+          </View>
+        </View>
+      </TouchableWithoutFeedback>
+    );
+  },
+  
+  _handlePress() {
+    this.setState({flipped: !this.state.flipped});
+  }
+});
+
 exports.title = '<View>';
 exports.description = 'Basic building block of all UI, examples that ' +
   'demonstrate some of the many styles available.';
@@ -188,5 +248,10 @@ exports.examples = [
         </View>
       );
     },
+  }, {
+    title: 'ZIndex',
+    render: function() {
+      return <ZIndexExample />;
+    },
   },
 ];