Use an InputEventSpy to notify about key and modifier state changes
Summary:
Instead of emitting the key state changed and modifier state changed
signals from the right point before processing the events, let's use
an InputEventSpy to do that. The spies were introduced to be called
directly before the event processing. So the contract still holds.
Reviewers: KWin, Plasma on Wayland
Subscribers: plasma-devel, kwin
Tags: Plasma on Wayland, KWin
Differential Revision: https://phabricator.kde.org/D4128