Add Window::SetContentProtection (exclude from screen capture)

[lijy91]

Add Window::SetContentProtection(bool) so the window's content is excluded from screenshots and screen recording, like Electron's win.setContentProtection():

• Windows: SetWindowDisplayAffinity(hwnd, WDA_EXCLUDEFROMCAPTURE) (Windows 10 2004+, else WDA_MONITOR)
• macOS: NSWindow.sharingType = NSWindowSharingNone (note: not honoured by ScreenCaptureKit on recent macOS)
• Linux: not available; document it

Background:

• Electron's win.setContentProtection(true); equivalent in flutter desktop? leanflutter/window_manager#215: equivalent of Electron's setContentProtection