kf6: Add a kf6 VCL plugin

Add a "kf6" VCL plugin that uses the KF6 (KDE Frameworks 6)
libraries to provide a native KDE/Plasma file chooser,
just like the kf5 VCL plugin does for KF5.

Building the plugin is disabled by default and can be enabled by
autogen option '--enable-kf6'.

Selecting the VCL plugin can be done by starting LO
with environment variable 'SAL_USE_VCLPLUGIN=kf6' set.

The kf6 VCL plugin reuses the kf5 VCL plugin code.
(The kf6 headers and sources for now just `#include`
the kf5 ones.)

This was quickly tested on KDE Neon unstable,
which provides a daily snapshot of Plasma 6 and the KF6
libraries.
(Regarding a potential release date, [1] mentions:
"Plasma 6 is built on top of Qt 6 and is
tentatively planned to be released in late 2023 or early 2024.")

[1] https://community.kde.org/Plasma/Plasma_6

Change-Id: I4c2b7e3be8e60f1d8cf60119f6f3f642b71349f5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/153438
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

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