Add Qt6 support for WindowsHEADmaster

This patch makes it possible to build LibreOffice with Qt6 VCL plugin on
Windows.

In the configuration stage, it is required to add --enable-qt6 to the
configuration options, and also set the QT6DIR to the place where Qt6 is
installed:

export QT6DIR=/cygdrive/c/qt/6.10.2/msvc2022_64

When building with --enable-headless in addition, Cairo
based rendering is used by default, but the QFont based rendering
can explicitly be enabled by setting environment variable
SAL_VCL_QT_USE_QFONT=1 when starting LibreOffice, as is the case on
Linux.
When using the QFont based rendering, two asserts related
to logic related to alpha/transparency support not fully being implemented
for that rendering path yet are triggered (in QtVirtualDevice::SetSize
and QtGraphicsBackend::getBitmap). They can be commented for
further experiments. That also matches the behavior seens on
Linux.

For a debug build of Qt, the file names of the libraries include
an additional "d" in fron of the ".lib" file extension, e.g. a file
name of "Qt6Widgetsd.lib" instead of just "Qt6Widgets.lib",
so support both variants in configure.

This is partially based on a previous commits from jmux for Qt5 in 2018.
One may see the old patches using:

$ git checkout remotes/origin/feature/qt5-win+mac

A subsequent patch is planned to add Qt6 support for macOS.

Co-authored-by: Jan-Marek Glogowski <glogow@fbihome.de>
Co-authored-by: Michael Weghorn <m.weghorn@posteo.de>
Change-Id: I42a3059c738fa95464fa49ae4aab1c9f686fee46
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/199615
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Reviewed-by: Hossein <hossein@libreoffice.org>

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