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

shell_client.cpp

@@ -985,6 +985,9 @@ bool ShellClient::isInternal() const
 
 bool ShellClient::isLockScreen() const
 {
+    if (m_internalWindow) {
+        return m_internalWindow->property("org_kde_ksld_emergency").toBool();
+    }
     return surface()->client() == waylandServer()->screenLockerClientConnection();
 }