[iOS] Added ability to set static(root) background image (#1366)

[iOS] Added ability to set static(root) background image

docs/styling-the-navigator.md

   
   disabledBackGesture: false, // default: false. Disable the back gesture (swipe gesture) in order to pop the top screen. 
   screenBackgroundImageName: '<name of image in Images.xcassets>', // Optional. default screen background image.
-  
+  rootBackgroundImageName: '<name of image in Images.xcassets>', // Static while you transition between screens. Works best with screenBackgroundColor: 'transparent'
+
   navBarButtonFontSize: 20, // Change font size nav bar buttons (eg. the back button) (remembered across pushes)
   navBarButtonFontWeight: '500', // Change font weight nav bar buttons (eg. the back button) (remembered across pushes)
 

ios/RCCViewController.m

     [[RCCManager sharedInstance] registerController:controller componentId:componentId componentType:type];
   }
   
+  // set background image at root level
+  NSString *rootBackgroundImageName = props[@"style"][@"rootBackgroundImageName"];
+  if (rootBackgroundImageName) {
+    UIImage *image = [UIImage imageNamed: rootBackgroundImageName];
+    UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
+    [controller.view insertSubview:imageView atIndex:0];
+  }
+  
   return controller;
 }