Replaced InputEventFilter by InputEventSpy in DebugConsole

Summary:
As explained in 84e33081499fd0049fbe7908dd7b66b2eefd34b1
the DebugConsole should use an InputEventSpy instead of an InputEventFilter.

Reviewers: kwin, graesslin

Reviewed By: graesslin

Subscribers: graesslin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3879