Window occlusion state and event #74

Apps sometimes need to know whether their window is actually visible to the user, not just shown but covered by other windows, for example to pause rendering or video.

Proposal: an occlusion state and event. macOS has it natively (NSWindow.occlusionState / NSWindowDidChangeOcclusionStateNotification); on Windows it can be approximated (DwmGetWindowAttribute(DWMWA_CLOAKED) plus a z-order/visible-region check), and on Linux via visibility-notify-event on X11 only.

Background:

• Can I listen to the current window being blocked leanflutter/window_manager#459: listen for the window being covered by another window