lib: support loading non-native GVfs URIs

Even though libview has EvJobLoadGFile, it doesn't really work. eg.,
PDFs fail to load over WebDAV. So, we download the files and convert
them to PDF using unoconv, if needed, before showing them.

https://bugzilla.gnome.org/show_bug.cgi?id=686527
documents: implement OwncloudDocument

https://bugzilla.gnome.org/show_bug.cgi?id=686527
miners: Add a JS stub for org.gnome.OnlineMiners.OwncloudMiner

https://bugzilla.gnome.org/show_bug.cgi?id=686527
miners: refresh the ownCloud cache on startup

https://bugzilla.gnome.org/show_bug.cgi?id=686527
documents: check the nao:identifier and instantiate a OwncloudDocument

https://bugzilla.gnome.org/show_bug.cgi?id=686527
properties: show an appropriate source for OwncloudDocuments

The presentation-identity from GOA is shown as the source. We could
have also shown the GVfs URI as the source, but I guess, we don't want
to promote the file manager any more than we need to.

https://bugzilla.gnome.org/show_bug.cgi?id=686527