GduVolumeGrid: Add basic accessibility

I bet this is still almost impossible to use, if you don't know to use
the arrow keys to move around the partition table, but at least now you
can hear what the partions are, you have some chance. Further
improvement probably needed.

P.S. Yes, this is a hack. But it works just fine in Orca.

https://bugzilla.gnome.org/show_bug.cgi?id=747490