Apple: Trace CFRunLoop activities

Add two run loop observers that:
- Create a TRACE-level span when the run loop enters a new state.
- Drops the span when the run loop exits that state.

These spans attach information to events, such that e.g. resizing a view
produces messages like:
```
TRACE inside runloop{mode=NSEventTrackingRunLoopMode}:timers:
  winit_appkit::util: Triggered `drawRect:` target="winit_appkit::view"
TRACE inside runloop{mode=NSEventTrackingRunLoopMode}:timers:
  winit_appkit::util: Completed `drawRect:` target="winit_appkit::view"
```

winit-uikit/src/event_loop.rs

@@ -12,7 +12,7 @@ use objc2_ui_kit::{
     UIApplicationWillResignActiveNotification, UIApplicationWillTerminateNotification, UIScreen,
 };
 use rwh_06::HasDisplayHandle;
-use winit_common::core_foundation::{MainRunLoop, MainRunLoopObserver};
+use winit_common::core_foundation::{MainRunLoop, MainRunLoopObserver, tracing_observers};
 use winit_core::application::ApplicationHandler;
 use winit_core::cursor::{CustomCursor, CustomCursorSource};
 use winit_core::error::{EventLoopError, NotSupportedError, RequestError};
@@ -134,6 +134,7 @@ pub struct EventLoop {
     _will_terminate_observer: Retained<ProtocolObject<dyn NSObjectProtocol>>,
     _did_receive_memory_warning_observer: Retained<ProtocolObject<dyn NSObjectProtocol>>,
 
+    _tracing_observers: Option<(MainRunLoopObserver, MainRunLoopObserver)>,
     _wakeup_observer: MainRunLoopObserver,
     _main_events_cleared_observer: MainRunLoopObserver,
     _events_cleared_observer: MainRunLoopObserver,
@@ -226,12 +227,19 @@ impl EventLoop {
         let main_loop = MainRunLoop::get(mtm);
         let mode = unsafe { kCFRunLoopDefaultMode }.unwrap();
 
+        // Tracing observers have the lowest and highest orderings.
+        let _tracing_observers = tracing_observers(mtm).inspect(|(start, end)| {
+            main_loop.add_observer(start, mode);
+            main_loop.add_observer(end, mode);
+        });
+
         let _wakeup_observer = MainRunLoopObserver::new(
             mtm,
             CFRunLoopActivity::AfterWaiting,
             true,
-            // Queued with the highest priority to ensure it is processed before other observers.
-            CFIndex::MIN,
+            // Queued with the second-highest priority (tracing observers use the highest) to
+            // ensure it is processed before other observers.
+            CFIndex::MIN + 1,
             move |_| app_state::handle_wakeup_transition(mtm),
         );
         main_loop.add_observer(&_wakeup_observer, mode);
@@ -241,17 +249,17 @@ impl EventLoop {
             CFRunLoopActivity::BeforeWaiting,
             true,
             // Core Animation registers its `CFRunLoopObserver` that performs drawing operations in
-            // `CA::Transaction::ensure_implicit` with a priority of `0x1e8480`. We set the
+            // `CA::Transaction::ensure_implicit` with a priority of `2000000`. We set the
             // main_end priority to be 0, in order to send `AboutToWait` before `RedrawRequested`.
             // This value was chosen conservatively to guard against apple using different
             // priorities for their redraw observers in different OS's or on different devices. If
             // it so happens that it's too conservative, the main symptom would be non-redraw
             // events coming in after `AboutToWait`.
             //
-            // The value of `0x1e8480` was determined by inspecting stack traces and the associated
+            // The value of `2000000` was determined by inspecting stack traces and the associated
             // registers for every `CFRunLoopAddObserver` call on an iPad Air 2 running iOS 11.4.
             //
-            // Also tested to be `0x1e8480` on iPhone 8, iOS 13 beta 4.
+            // Also tested to be `2000000` on iPhone 8, iOS 13 beta 4.
             0,
             move |_| app_state::handle_main_events_cleared(mtm),
         );
@@ -261,8 +269,9 @@ impl EventLoop {
             mtm,
             CFRunLoopActivity::BeforeWaiting,
             true,
-            // Queued with the lowest priority to ensure it is processed after other observers.
-            CFIndex::MAX,
+            // Queued with the second-lowest priority (tracing observers use the lowest) to ensure
+            // it is processed after other observers.
+            CFIndex::MAX - 1,
             move |_| app_state::handle_events_cleared(mtm),
         );
         main_loop.add_observer(&_events_cleared_observer, mode);
@@ -277,6 +286,7 @@ impl EventLoop {
             _did_enter_background_observer,
             _will_terminate_observer,
             _did_receive_memory_warning_observer,
+            _tracing_observers,
             _wakeup_observer,
             _main_events_cleared_observer,
             _events_cleared_observer,