qpa: Provide a QPlatformNativeInterface

Under Gnome/Unity/XFCE, but not Plasma(!), QFontconfigDatabase queries
the platform native interface for certain font settings
(e.g. antialiasingEnabled). Without a QPNI this would crash KWin
as soon as it tried to query for fonts.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>
Part-of: <!2468>


(cherry picked from commit b018dd32)