API-CHANGE - dropping string properties which use vnd.sun.star.GraphicObject URL

[Tomaž Vajngerl]

Hi,

I'm working on image handling re-work [1] [2] [3] and I need to drop
the GraphicURL (and similar properties like BackGraphicURL - see [3]
and most of the changes should be contained in [4] patch) of type
String and use a property of the type XGraphic. The reason is to make
reference-counting safe, which means that exposing internal URL's
(URLs with prefix vnd.sun.star.GraphicObject) will go away and
XGraphic will be used instead. Most of the details are written in [2]
and [3] so I won't write it here again.

[1] https://wiki.documentfoundation.org/Development/Budget2017#Image_handling_re-work
[2] https://tomazvajngerl.blogspot.com/2018/01/improving-image-handling-in-libreoffice.html
[3] https://tomazvajngerl.blogspot.com/2018/03/improving-image-handling-in-libreoffice.html
[4] https://gerrit.libreoffice.org/51201

Best Regards, Tomaž