Split Keyboard Repeat handling into a dedicated InputEventSpy

Summary:
So far the keyboard repeat handling was triggered directly from
KeyboardInputRedirection::processKey. With the introduction of
InputEventSpies it is no longer required to be done like that, we can
split it out into a dedicated spy.

This means that processKey only has to care about processing the key
and allows us to better extend in future. So far keyboard repeat is
only functional for libinput based platforms. But it should also be
possible to use it in nested setups. By splitting it out we can
prepare for that.

Test Plan: Auto-test using repeat still passes

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D4304