[LyX/master] Enable QWindowsMimeMetafile with Qt6

[Enrico Forestieri]

On Sun, Oct 17, 2021 at 02:48:31PM +0200, Enrico Forestieri wrote:
> commit 771f30e9463d8d7bf7acd4dfc564f462b28eae16
> Author: Enrico Forestieri <forenr at lyx.org>
> Date:   Sun Oct 17 15:06:05 2021 +0200
> 
>     Enable QWindowsMimeMetafile with Qt6
>     
>     The QWinMime class has been removed in Qt6 but the functionality
>     is still present. However, one has to allow inclusion of private
>     headers and register the mime handling to the QWindowsApplication
>     native interface.

Kornel, according to
https://www.qt.io/blog/qt-extras-modules-in-qt-6#qwinmime
to enable private headers with cmake one has to add a project dependency
to Qt::GuiPrivate. I honestly don't know how to do that. This only
matters for Windows and after this commit compilation cannot succeed
when configuring with cmake if private headers are not enabled.

-- 
Enrico