Re-introduce WM_CLOSE listener, new quit protocol posts a second message to next handler (#40802)

With this approach, when the last window of an app is closed, the engine
sends a message to the framework to check if it is allowed to exit. If
it may, the windows lifecycle manager synthesizes a second `WM_CLOSE`
message that it will ignore, and so the next registered top level window
proc delegate, if any, will be able to process the message. If none do
so, the message will be handled by the default window proc, so the app
will be able to close.

I was not able to get a full system tray example application running to
test this, but I could get an application that stays open when its
window is closed and can be seen as a system tray icon as long as it is
running, albeit the icon was non-functional. As this repro app still
exhibited this behavior when using this engine build, I am reasonably
confident in concluding that applications that want to be able to run
headless when their windows close will function properly.

Addresses https://github.com/flutter/flutter/issues/123654.

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---------

Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>