gtkplacesviewrow: plural form for available space
gtkplacesviewrow: plural form for available space

We were not supporting plural form of the available space, which
is a problem in some languages.
However in this case is kind of a difficult matter, since we use a
formatted string from glib with g_format_size.

To fix it, use the same behavior as g_format_size to decide when
it should be used a plural form or not.

https://bugzilla.gnome.org/show_bug.cgi?id=759491

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita