Pass SwitchEvent from LibInput through KWin and add to DebugConsole
This change introduces a new SwitchEvent and passes it through the
InputEventSpy and InputEventFilter. The DebugConsoleFilter implements it
so that the events can be monitored in the debug console.
Test Plan: Untested as my only device with such switches has too old libinput
Subscribers: plasma-devel, kwin
Differential Revision: https://phabricator.kde.org/D9521