[libkwineffects] Save value of the managed property during construction of EffectWindow

Summary:
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.

Test Plan: Manually.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13690