Windows: Make EventLoopWindowTarget independent of UserEvent type (#3061)

* make `EventLoopWindowTarget` independent of UserEvent type

the `EventLoopWindowTarget` is needed for window creation. conceptually,
only `EventLoop` and `EventLoopProxy` need to be parameterized, and all
other parts of the backend should be agnostic about the user event type,
parallel to how `Event<T>` is parameterized, but `WindowEvent` is not.

this change removes the dependency on the type of user events from the
`EventLoopWindowTarget` for the Windows backend, but keep a phantom data
to keep the API intact. to achieve this, I moved the `Receiver` end of
the mpsc channel from `ThreadMsgTargetData` into `EventLoop` itself, so
the `UserEvent` is only passed between `EventLoop` and `EventLoopProxy`,
all other part of the backend just use unit type as a placeholder for
user events.

it's similar to the macos backend where an erased `EventHandler` trait
object is used so all component except `EventLoop` and `EventLoopProxy`
need to be parameterized. however `EventLoop` of the Windows backend
already use an `Box<dyn FnMut>` to wrap the user provided event handler
callback, so no need for an dedicated trait object, I just modified the
wrapper to replace the placeholder user event with real value pulled
from the channel. I find this is the approach which need minimum change
to be made to existing code. but it does the job and could serve as a
starting point to future Windows backend re-works.

* fix CI clippy failure.

* make UserEventPlaceholder a new type instead of alias

* invariance is maintained by top-level EventLoopWindowTarget<T>

this field is transitional and her to keep API compatibility only.
the correct variance and such is already ensured by the top-level
`EventLoopWindowTarget`, just use `PhantomData<T>` here.

src/platform_impl/windows/event_loop/runner.rs

@@ -416,7 +416,7 @@ impl<T> BufferedEvent<T> {
 
                 let window_flags = unsafe {
                     let userdata =
-                        get_window_long(window_id.0.into(), GWL_USERDATA) as *mut WindowData<T>;
+                        get_window_long(window_id.0.into(), GWL_USERDATA) as *mut WindowData;
                     (*userdata).window_state_lock().window_flags
                 };
                 window_flags.set_size((window_id.0).0, inner_size);