Add unregisterForPreviewingWithContext (#2239)

* Preview API

* Added preview actions and fail-guard wrappers

* Remove old setters

* Moved from passProps to actionParams

* Move findNodeHandle to internal method

* Removing findNodeHandle

* Added height and option to dont 'pop' the view controller

* Documentation update for peek and pop

* Document how to access button press events

* Commit by default in example. Because it's cool

* Docs and if elses ifs then

* Unregister previous ViewController on attempt to preview.

* Merge conflict fail

ios/RCCNavigationController.m

       if ([self.topViewController isKindOfClass:[RCCViewController class]])
       {
         RCCViewController *topViewController = ((RCCViewController*)self.topViewController);
+        topViewController.previewController = nil;
+        [topViewController.navigationController unregisterForPreviewingWithContext:topViewController.previewContext];
         viewController.previewActions = previewActions;
         viewController.previewCommit = actionParams[@"previewCommit"] ? [actionParams[@"previewCommit"] boolValue] : YES;
         NSNumber *previewHeight = actionParams[@"previewHeight"];
             [bridge.uiManager addUIBlock:^(__unused RCTUIManager *uiManager, NSDictionary<NSNumber *, UIView *> *viewRegistry) {
               UIView *view = viewRegistry[previewViewID];
               topViewController.previewView = view;
-              [topViewController registerForPreviewingWithDelegate:(id)topViewController sourceView:view];
+              topViewController.previewContext = [topViewController registerForPreviewingWithDelegate:(id)topViewController sourceView:view];
             }];
           });
           topViewController.previewController = viewController;

ios/RCCViewController.h

 @property (nonatomic) UIView *previewView;
 @property (nonatomic) NSArray *previewActions;
 @property (nonatomic) BOOL previewCommit;
+@property (nonatomic) id previewContext;
 
 + (UIViewController*)controllerWithLayout:(NSDictionary *)layout globalProps:(NSDictionary *)globalProps bridge:(RCTBridge *)bridge;