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 m...