Support the emergency show property on ksld's lock screen window

Summary:
The emergency show window should be considered as a lock screen window
and be shown while the screen is locked. This is the KWin part to D5157.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5158