Update objc2 crates (#3634)

Changes relevant to Winit:
- `icrate` has been deprecated in favour of separate crates per
  framework, in our case `objc2-foundation` and `objc2-app-kit` (and in
  the future `objc2-ui-kit` on iOS).
- Moved `MainThreadMarker::run_on_main` to free-standing function
  `run_on_main`.
- Changed how features work, this should result in less code that we
  need to compile.
- Enums are now real structs instead of type-aliases and free constants.

src/platform_impl/macos/event_loop.rs

@@ -16,16 +16,13 @@ use core_foundation::runloop::{
     kCFRunLoopCommonModes, CFRunLoopAddSource, CFRunLoopGetMain, CFRunLoopSourceContext,
     CFRunLoopSourceCreate, CFRunLoopSourceRef, CFRunLoopSourceSignal, CFRunLoopWakeUp,
 };
-use icrate::AppKit::{
-    NSApplication, NSApplicationActivationPolicyAccessory, NSApplicationActivationPolicyProhibited,
-    NSApplicationActivationPolicyRegular, NSWindow,
-};
-use icrate::Foundation::{MainThreadMarker, NSObjectProtocol};
 use objc2::{msg_send_id, ClassType};
 use objc2::{
     rc::{autoreleasepool, Id},
     runtime::ProtocolObject,
 };
+use objc2_app_kit::{NSApplication, NSApplicationActivationPolicy, NSWindow};
+use objc2_foundation::{MainThreadMarker, NSObjectProtocol};
 
 use super::event::dummy_event;
 use super::{
@@ -232,9 +229,9 @@ impl<T> EventLoop<T> {
         }
 
         let activation_policy = match attributes.activation_policy {
-            ActivationPolicy::Regular => NSApplicationActivationPolicyRegular,
-            ActivationPolicy::Accessory => NSApplicationActivationPolicyAccessory,
-            ActivationPolicy::Prohibited => NSApplicationActivationPolicyProhibited,
+            ActivationPolicy::Regular => NSApplicationActivationPolicy::Regular,
+            ActivationPolicy::Accessory => NSApplicationActivationPolicy::Accessory,
+            ActivationPolicy::Prohibited => NSApplicationActivationPolicy::Prohibited,
         };
         let delegate = ApplicationDelegate::new(
             mtm,