(allow-none), though deprecated since GI 1.42, is required to
maintain the backward compatibility since (nullable), added in
GI 1.42, is not understood by GI tools < 1.42 preventing the
clients from passing a null parameter to various functions in the
widget.

We can remove this deprecated (allow-none) annotation once we
bump the GI version in LibreOffice to atleast 1.42

Change-Id: I98a1f3d2205ec5afd8060f16e69c5f938f229e26