Allow the user to register the application delegate on macOS and iOS (#3758)

This allows the user to override the application delegate themselves,
which opens several doors for customization that were previously closed.

To do this, we use notifications instead of top-level application delegate
methods.

One effect of not providing an application delegate on iOS is that we no
longer act as-if the application successfully open all URLs there.

This is a breaking change, although unlikely to matter in practice, since the
return value of `application:didFinishLaunchingWithOptions:` is seldom used by
the system (and is likely the preferred behaviour anyhow).

Cargo.toml

@@ -105,12 +105,12 @@ ndk = { version = "0.9.0", default-features = false }
 
 # AppKit or UIKit
 [target.'cfg(target_vendor = "apple")'.dependencies]
+block2 = "0.5.1"
 core-foundation = "0.9.3"
 objc2 = "0.5.2"
 
 # AppKit
 [target.'cfg(target_os = "macos")'.dependencies]
-block2 = "0.5.1"
 core-graphics = "0.23.1"
 objc2-app-kit = { version = "0.2.2", features = [
     "NSAppearance",
@@ -152,6 +152,7 @@ objc2-foundation = { version = "0.2.2", features = [
     "NSKeyValueObserving",
     "NSNotification",
     "NSObjCRuntime",
+    "NSOperation",
     "NSPathUtilities",
     "NSProcessInfo",
     "NSRunLoop",
@@ -163,11 +164,13 @@ objc2-foundation = { version = "0.2.2", features = [
 # UIKit
 [target.'cfg(all(target_vendor = "apple", not(target_os = "macos")))'.dependencies]
 objc2-foundation = { version = "0.2.2", features = [
+    "block2",
     "dispatch",
     "NSArray",
     "NSEnumerator",
     "NSGeometry",
     "NSObjCRuntime",
+    "NSOperation",
     "NSString",
     "NSProcessInfo",
     "NSThread",