Only repeat one key

Summary:
So far KWin tried to repeat all pressed keys which should repeat. But
this is not how X11 and e.g. QtWayland handle it. There only one key -
the last one which got pressed - repeats. And this makes sense as the
key is used to generate a keysym and that one KWin caches. Thus the
logic so far resulted in incorrect keysyms to be generated during the
repeat. E.g. pressing a, pressing b, releasing b would repeat b instead
of the hold a as b was the last generated keysym.

This change addresses this problem and let's only one key repeat at a
time. When the currently repeating key gets released the repeat timer is
stopped and other hold keys won't repeat any more. This also matches the
behavior of X11 and QtWayland.

BUG: 369091
FIXED-IN: 5.8.1

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2941