Fix Logout screen countdown not cancelling on multi-monitor setups

Summary:
The logout greeter theme has a timer that invokes the current action if
no keyboard interaction has taken place.

This cancel on keyboard nav is broken on multiple monitor setups as even
if a user is interacting with one, the timer from another can still
fire.

Fix is via JS singleton library as the timer is very theme specific. I
didn't want to add anything in shutdowndlg.cpp as it would be breaking
layering.

BUG: 394249

Test Plan:
Had multi monitor setup
Pressed key on one screen. All timer UIs cleared, nothing happened after
30 seconds

Reviewers: #plasma

Subscribers: zzag, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12893