Create CSS rules for the attributes presentation:display-*

Improves the display of many of my test ODP slides :)

Also achieves pretty similar results in LO and the viewer for
https://wiki.gnome.org/GUADEC/2014/LightningTalks/SlideDesign?action=AttachFile&do=view&target=lightning_talks.odp

Once this is in and 0.5.2 out, they will have no excuse not to use ViewerJS for their webpages and even presentations :)
I suspect my brain is missing the obvious here (so apologies in advance), but why is a custom namespace and attribute used for this? Why don't you just set ``draw:style-name`` on the shadow content? This would make the ``Style2CSS`` logic below follow the currently relatively consistent behaviour of creating CSS rules limited by style-name.