To make it consistent, change "Graphic" property (added during
image handling rework) to "GraphicBitmap" in style::NumberingLevel,
which is already used in text::NumberingLevel. Both of them are
accessors for the graphic of bitmap bullets.

Change-Id: I70f7bccd80fa9949778c49e23de583c6fb32a796
Reviewed-on: https://gerrit.libreoffice.org/54492Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>