Added the ability to set an image title in the NavigatorIOS

Summary:
This pull request adds support for setting an image title in `NavigatorIOS`. It sets the `titleView` property of `UINavigationItem`.

Added an example into `UIExplorer`

<img width="487" alt="ex" src="https://cloud.githubusercontent.com/assets/289640/16147578/3d552606-3484-11e6-9a26-1e8c113ace03.png">

The code follows the same style as the rest of `RCTNavItem` and `RCTWrapperViewController`.
Closes https://github.com/facebook/react-native/pull/8182

Differential Revision: D3469073

Pulled By: javache

fbshipit-source-id: 2d3e4bad534f92b6e6b24a6b74f2993f3a8c9012

React/Views/RCTNavItem.h

@@ -14,6 +14,7 @@
 @interface RCTNavItem : UIView
 
 @property (nonatomic, copy) NSString *title;
+@property (nonatomic, strong) UIImage *titleImage;
 @property (nonatomic, strong) UIImage *leftButtonIcon;
 @property (nonatomic, copy) NSString *leftButtonTitle;
 @property (nonatomic, strong) UIImage *rightButtonIcon;
@@ -27,6 +28,7 @@
 @property (nonatomic, strong) UIColor *titleTextColor;
 @property (nonatomic, assign) BOOL translucent;
 
+@property (nonatomic, readonly) UIImageView *titleImageView;
 @property (nonatomic, readonly) UIBarButtonItem *backButtonItem;
 @property (nonatomic, readonly) UIBarButtonItem *leftButtonItem;
 @property (nonatomic, readonly) UIBarButtonItem *rightButtonItem;

React/Views/RCTNavItem.m

@@ -15,6 +15,15 @@
 @synthesize leftButtonItem = _leftButtonItem;
 @synthesize rightButtonItem = _rightButtonItem;
 
+- (UIImageView *)titleImageView
+{
+  if (_titleImage) {
+    return [[UIImageView alloc] initWithImage:_titleImage];
+  } else {
+    return nil;
+  }
+}
+
 - (void)setBackButtonTitle:(NSString *)backButtonTitle
 {
   _backButtonTitle = backButtonTitle;

React/Views/RCTNavItemManager.m

@@ -29,6 +29,7 @@ RCT_EXPORT_VIEW_PROPERTY(translucent, BOOL)
 
 RCT_EXPORT_VIEW_PROPERTY(title, NSString)
 RCT_EXPORT_VIEW_PROPERTY(titleTextColor, UIColor)
+RCT_EXPORT_VIEW_PROPERTY(titleImage, UIImage)
 
 RCT_EXPORT_VIEW_PROPERTY(backButtonIcon, UIImage)
 RCT_EXPORT_VIEW_PROPERTY(backButtonTitle, NSString)

React/Views/RCTWrapperViewController.m

@@ -123,6 +123,7 @@ static UIView *RCTFindNavBarShadowViewInView(UIView *view)
 
     UINavigationItem *item = self.navigationItem;
     item.title = _navItem.title;
+    item.titleView = _navItem.titleImageView;
     item.backBarButtonItem = _navItem.backButtonItem;
     item.leftBarButtonItem = _navItem.leftButtonItem;
     item.rightBarButtonItem = _navItem.rightButtonItem;