Refactor usage of CFRunLoopObserver (#4349)

Added a common interface that:
- Uses closures instead of static functions. This should allow easier
  refactoring in the future.
- Returns a handle which is invalidated on `Drop`. This should avoid
  situations where the event loop has exited, but an observer is still
  called because the user spawned the application later on.
- Is properly main-thread safe.

This interface is placed in winit-common, to allow using it in both
winit-appkit and winit-uikit.

winit-common/Cargo.toml

@@ -18,7 +18,7 @@ x11 = ["xkbcommon-dl?/x11", "dep:x11-dl"]
 xkb = ["dep:xkbcommon-dl", "dep:smol_str"]
 
 # CoreFoundation
-core-foundation = ["dep:objc2", "dep:objc2-core-foundation"]
+core-foundation = ["dep:block2", "dep:objc2", "dep:objc2-core-foundation"]
 
 [dependencies]
 smol_str = { workspace = true, optional = true }
@@ -31,9 +31,12 @@ x11-dl = { workspace = true, optional = true }
 xkbcommon-dl = { workspace = true, optional = true }
 
 # CoreFoundation
+block2 = { workspace = true, optional = true }
 objc2 = { workspace = true, optional = true }
 objc2-core-foundation = { workspace = true, optional = true, features = [
     "std",
+    "block2",
+    "objc2",
     "CFRunLoop",
     "CFString",
 ] }