macOS: Improve event queuing (#3708)

* Use AppKit's internal queuing mechanisms

This allows events to be queued in a more consistent order, they're now
interleaved with events that we handle immediately (like redraw events),
instead of being handled afterwards.

* Only queue events if necessary

This makes the call stack / backtraces easier to understand whenever
possible, and generally improves upon the order in which events are
delivered.

Cargo.toml

@@ -112,10 +112,12 @@ objc2 = "0.5.2"
 
 [target.'cfg(target_os = "macos")'.dependencies]
 core-graphics = "0.23.1"
+block2 = "0.5.1"
 
 [target.'cfg(target_os = "macos")'.dependencies.objc2-foundation]
 version = "0.2.2"
 features = [
+    "block2",
     "dispatch",
     "NSArray",
     "NSAttributedString",