Avoid event processing infinite loop

Summary:
Kwin on X11 overrides QCoreApplication::notify to get low level system
event.

If something handled by the low level filtering ever calls postEvent we
would end up in a loop. This was found when forwarding a key event into
a QQuickWindow in a similar manner that mouse events are forwarded.

Fortunately there's a flag to separate generated and core events

Test Plan:
No more infinite loop
Kwin X11 still works fine

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24207