/usr/share/kde4/apps/kwin/default_rules/plasma_desktop_containment.kwinrules is in kde-window-manager-common 4:4.11.8-0ubuntu6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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count=1
[1]
Description=Add Widgets dialog appears near the mouse cursor
placement=UnderMouse
placementrule=2
title=Add Widgets
titlematch=1
types=32
wmclass=plasma
wmclasscomplete=false
wmclassmatch=1
|