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 :)
Is this going to naturally have a default value of visible if the attribute does not exist? If so, removing one of the if-checks would help clarify the default state.