fix saving window settings

we should not save settings from hidden windows.
also should we respect cancellation
update 1.0.0.md
extend flyout demo for testing window settings
events, events, events

- move deregistering events to closed event
- handle minimized state changed, because hidden windows gets no window
placements
fix demo
i think this assert can be placed after the hooked check
add this for the assert changes
Merge pull request #1672 from MahApps/1671-punker76-WindowSettings-fixes

fix for #1671 saving window settings