[Windows] Verify: flashes and blank frames on start, show, restore and maximize

Start with a Flutter app using the Window API and reproduce each of the eight linked Windows reports against nativeapi 0.6.0. Check startup, show, restore, maximize, positioning, and repaint behavior, including the runner's first-frame Show() race. Done means each checklist item is marked as reproduced and fixed or documented as no longer occurring.

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.

• leanflutter/window_manager#153: black background flashes when restoring from maximized
• leanflutter/window_manager#155: white screen for up to seconds when showing after hide()
• leanflutter/window_manager#383: white screen after maximize()
• leanflutter/window_manager#428: white flash on startup before the background colour applies
• leanflutter/window_manager#504: window flashes at another position before moving to the centre on startup
• leanflutter/window_manager#578: part of a window dragged back from off-screen isn't repainted in time
• leanflutter/window_manager#412: maximize() at startup reverts to the default size (runner's first-frame Show() races with it)
• leanflutter/window_manager#223 / leanflutter/window_manager#572: position or maximize the main window before it first appears, without a visible jump