Improve PowerOff/PowerDown behaviour

Summary:
Trigger PowerDown after pressing for 1s instead of having to wait for release
to decide, feels more natural.
Also don't operate the modifiers, it's done later by KGlobalAccel.

Reviewers: KWin, Plasma:_mobile, bshah

Reviewed By: Plasma:_mobile, bshah

Subscribers: ngraham, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28490