When windowClosed signal is emitted, effects can't distinguish managed
windows from unmanaged windows(e.g. combo box popups, popup menus, etc).
This leads to dirty hacks like IsXXXWindow. Also, there's a big chance
that such hack can introduce more bugs and overall this makes harder to
This change proposes to save value of managed property during
construction of EffectWindow. So, its value is preserved with Deleted.