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
write/maintain effects.
This change proposes to save value of managed property during
construction of EffectWindow. So, its value is preserved with Deleted.