When LOOL requires to insert a graphic, it uses UNO command ".uno:InsertGraphic", 
and the corresponding URL file image argument.

However Impress, ignores URL file image argument, and by default
pop up a dialog, so no image is inserted in LOOL.

Change-Id: If1939247f5df0bb4e7daf8816a2306e34419ae2f
Reviewed-on: https://gerrit.libreoffice.org/20747Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Henry Castro <hcastro@collabora.com>