Send a Qt::Key_PowerDown when long pressing the power button

Summary:
This will be useful to be able to show a menu on long press and just
lock the screen on normal press, especially useful on phones and
tablets.

Test Plan:
Could not test much because on laptops doesn't work. Would need more
investigation. discussed further on the plasma mailing list.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: z3ntu, davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26899