Split modifier only handling into a dedicated InputEventSpy

Summary:
The functionality regarding triggering modifier only shortcuts is moved
out of Xkb - where it doesn't belong to - and is turned into an input
event spy listening for the changes it is interested in. Previously
the state got queried by asking e.g. for the pressed buttons, now it's
tracked directly.

The X11 side needs a larger change due to that as now pushing the events
into Xkb does not trigger modifier only shortcuts any more. Instead the
"normal" way through the platform API needs to be used which triggers the
processing of filters and spies.

The problem here is that our redirections only process events if they are
inited and that only happens on Wayland. We cannot call init on them as
that would create all the Wayland filters and spies and processing would
probably break. As an intermediate solution the spies are now processed
and there we know that it won't matter. A future solution would be to
remove the init checks completely and just send through both filters and
spies and ensure that on X11 only the supported ones are loaded.

Closes T5220

Test Plan: Tested on Wayland and X11

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Maniphest Tasks: T5220

Differential Revision: https://phabricator.kde.org/D4578