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