placesview: implement available space

placesview: implement available space

GtkPlacesView is a widget to display locations
in the computer, such as root ("/") and volumes,
separating the persistent devices from removable
ones.

From the latest mockups[1], GtkPlacesView would
display the available space of local drives like
partitions. This, however, is not implemented in
the current codebase.

Fix that by implementing the measurement of disk
space, and adding a new property GtkPlacesView::show-disk-usage
which controls the visibility of measured disks.

[1] https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/nautilus/nautilus-next/other-locations.png

https://bugzilla.gnome.org/show_bug.cgi?id=759225