[Windows] Verify: full-screen transitions

[lijy91]

These were reported against window_manager 0.5.x, whose platform code is gone: window_manager 0.6.0 is rebuilt on nativeapi. None of them has been checked against nativeapi yet. Each needs a reproduction with nativeapi (a Flutter app on the Window API), then a fix or a note that it no longer happens. Tick them off here as they're checked.

Related: #58 (full-screen changed event).

• The fullScreen is not functioning properly. leanflutter/window_manager#458: after starting in full screen, setFullScreen(false) shows only the background colour; the taskbar icon disappears and minimize stops working
• On Windows, title bar not appearing after coming back from full screen leanflutter/window_manager#579: title bar doesn't come back after leaving full screen
• windowManager.setTitleBarStyle(TitleBarStyle.normal) should update window style to WS_OVERLAPPED leanflutter/window_manager#569: a window created with WS_POPUP doesn't get its caption back when leaving full screen / setting TitleBarStyle.normal
• Fullscreen problem: previous screen visible at the bottom when pushing new screen leanflutter/window_manager#330: previous content visible at the bottom when switching to or from full screen (video player)