Swizzle sendEvent: instead of subclassing NSApplication (#4036)

This is done to avoid order-dependent behavior that you'd otherwise
encounter where `EventLoop::new` had to be called at the beginning of
`fn main` to ensure that Winit's application was the one being
registered as the main application by calling `sharedApplication`.

Fixes https://github.com/rust-windowing/winit/issues/3772.

This should also make it (more) possible to use multiple versions of
Winit in the same application (though that's still untested).

Finally, it should allow the user to override `NSApplication` themselves
if they need to do that for some reason.

src/changelog/unreleased.md

@@ -185,6 +185,7 @@ changelog entry.
 - Updated `windows-sys` to `v0.59`.
   - To match the corresponding changes in `windows-sys`, the `HWND`, `HMONITOR`, and `HMENU` types
     now alias to `*mut c_void` instead of `isize`.
+- On macOS, no longer need control of the main `NSApplication` class (which means you can now override it yourself).
 
 ### Removed