Commit dfee7d93 (sw: get rid of
FN_UNO_GRAPHIC_U_R_L and "GraphicURL" property, 2018-03-08) removed
support for string-based graphic references, then a later commit
restored the setter. But one scenario (when setting the graphic URL
before inserting the text graphic object) was still unsupported for real
URL setter; restore that as well.

Change-Id: I52a7f96e820f614d9d031df3ba03c794984ad68b
Reviewed-on: https://gerrit.libreoffice.org/58669Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins