fix(compat): Set the app delegate viewController for plugins

Some plugins expect to be able to access the applications's
CDVViewController via the AppDelegate. While previously we guaranteed
that the `viewController` property was set to a non-nil
CDVViewController, there was never any guarantee that it was set to the
CDVViewController that is actually displaying the app (particularly in
cases such as apps using CordovaLib only for a few pages as part of a
larger app).

We've attempted to deprecate the property here, but some plugins still
rely on the assumption that it will be non-nil, so we are trying to
maintain compatibility in the common case here by assigning to it when a
CDVViewController is loaded. This means there are still times in the app
lifecycle where the `viewController` property is nil, but the
deprecation warning will hopefully spur plugin developers to move away
from that pattern.

Note: CDVPlugin always has a non-nil viewController property of its own,
which is set to the CDVViewController instance the plugin was loaded
into. This is almost always the better choice for plugins to use.

CordovaLib/Classes/Public/CDVViewController.m

@@ -295,6 +295,17 @@ - (void)viewDidLoad
 {
     [super viewDidLoad];
 
+    // TODO: Remove in Cordova iOS 9
+    if ([UIApplication.sharedApplication.delegate isKindOfClass:[CDVAppDelegate class]]) {
+        CDVAppDelegate *appDelegate = (CDVAppDelegate *)UIApplication.sharedApplication.delegate;
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+        if (appDelegate.viewController == nil) {
+            appDelegate.viewController = self;
+        }
+#pragma clang diagnostic pop
+    }
+
     // Load settings
     [self loadSettings];
 

CordovaLib/include/Cordova/CDVAppDelegate.h

@@ -41,7 +41,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nullable, nonatomic, strong) IBOutlet UIWindow *window API_DEPRECATED_WITH_REPLACEMENT("SceneDelegate:window", ios(2.0, 13.0));
 
 // TODO: Remove in Cordova iOS 9
-@property (nullable, nonatomic, strong) IBOutlet CDVViewController *viewController CDV_DEPRECATED(8, "This will always be nil.");
+@property (nullable, nonatomic, strong) IBOutlet CDVViewController *viewController CDV_DEPRECATED(8, "");
 
 @end