Add a `KWinWindowManager` that can be used to retrieve
window positions using the KWin window manager/Wayland
compositor that is used on KDE Plasma by default.

In order to do that, a JavaScript KWin script is used
that retrieves the global/screen positions of windows
using KWin's scripting API [1].
The script can be loaded and run via DBus.
(python3-dbus is used for this, added as a new dependency.)

Unfortunately, the script output is currently not
available via DBus, which seems to be a KDE bug,
see [2] [3] [4].

As a workaround, retrieve the script output
from the journal using `journalctl` instead, see
also the discussion in [4].

For now, the Wnck-based window manager (that doesn't
work on Wayland with native Wayland apps, see
issue #41) remains the default.
Use of the KWin window manager can explicitly
be enabled by setting the environment variable
`ACCERCISER_WINDOW_MANAGER=kwin`.

(Adding some heuristics to choose a WindowManager
might make sense at some point in the future once
this is more feature-complete.)

Using the new WindowManager in a KDE Plasma 5
Wayland session on Debian testing
(kwin-wayland at package version 4:5.27.10-1)
results in objects in the a11y tree view
being highlighted as expected in most, but not
all cases yet:

* Qt apps like the "standarddialogs" example
  from qtbase or the qt6-based LO variant
  work fine when run as a native Wayland
  application.

* When run on XWayland (forced via `QT_QPA_PLATFORM=xcb`)
  the qtbase "standarddialogs" sample highlighting
  is slightly incorrect for the initial window; the window
  frame/decoration doesn't seem to be taken into account properly,
  so the highlighting rectangle is a bit too far up.
  The actual dialogs that can be run in that sample
  are handled correctly again, though.

* GTK 4 application "transmission-gtk"
  (version 4.0.5-1) works fine.

* for (GTK 3) gedit (version 46.1-3), the
  area highlighted for the top-level frame is
  odd, but anything else looks fine.

* For gtk4-demo, the highlighted
  area is slightly off (maybe around 30
  pixels too far on the left and top).

This partly solves #41 by providing an initial
solution when KDE Plasma Wayland with KWin is used,
at least for the highlighting part.
More work is needed to make the quick select feature
using the mouse position work as well.

A similar approach should generally be possible for
GNOME/Mutter.

[1] https://develop.kde.org/docs/plasma/kwin/api/
[2] https://bugs.kde.org/show_bug.cgi?id=477069
[3] https://bugs.kde.org/show_bug.cgi?id=392840
[4] https://bugs.kde.org/show_bug.cgi?id=445058