Don't send keyboard events to QWindows which have outputOnly property set

Summary:
PlasmaCore.Dialog has a property outputOnly. This is for example used in
KWin's OnScreenNotification. If that property is set KWin should not send
any key events to it, just like for windows with _q_showWithoutActivating.

BUG: 388112

Test Plan: Not yet

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9618