rename Image Styles to Drawing Object Styles

Change-Id: Id0c5e9fb505ad99bb77f80963b30b8f0946cc48d
Reviewed-on: https://gerrit.libreoffice.org/11594
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>