Add a window full-screen changed event

Start by tracing Window::SetFullScreen() and IsFullScreen() on macOS, Windows, and Linux. Review the listed native notifications and window-state event, including the Windows style and title-bar restoration concerns. Done means a full-screen change event is emitted for programmatic and user-triggered changes on each platform.

Window::SetFullScreen() / IsFullScreen() exist on all three desktop platforms, but there is no event when a window enters or leaves full screen. That includes changes the user triggers (the green button on macOS, F11 / the window manager on Linux). Apps that change their layout in full screen currently have to poll.

Proposal: add a WindowFullScreenChangedEvent (or an entered/left pair), emitted:

• macOS: NSWindowDidEnterFullScreenNotification / NSWindowDidExitFullScreenNotification
• Windows: from SetFullScreen(), which is the only way in since Win32 has no native full-screen state. Note that WM_SIZE arrives with SIZE_RESTORED, so it can't be detected from the size message.
• Linux: window-state-event with GDK_WINDOW_STATE_FULLSCREEN

It should also be checked that the Windows SetFullScreen() restores the style and title bar correctly on exit (see leanflutter/window_manager#579 and #569).

Background:

• leanflutter/window_manager#560: onWindowEnterFullScreen / onWindowLeaveFullScreen never fire on Windows
• leanflutter/window_manager#559: a draft PR for Windows full-screen handling